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CP/M Basic Disk Operating System
The Basic Disk Operating System, or BDOS, provided access to such operations as opening a file, output to the console, or printing. Application programs would load processor registers with a function code for the operation, and addresses for parameters or memory buffers, and call a fixed address in memory. Since the address was the same independent of the amount of memory in the system, application programs would run the same way for any type or configuration of hardware.

CP/M Disk formats The fragmented CP/M market, requiring distributors either to stock multiple formats of disks or to invest in multiformat duplication equipment, compared with the more standardized IBM PC disk formats, was a contributing factor to the rapid obsolescence of CP/M after 1981.

Disk partitioning Disk partitioning is the act of dividing a hard disk drive (HDD) into multiple logical storage units referred to as partitions, to treat one physical disk drive as if it were multiple disks, so that different filesystems can be used on each partition.

Disk partitioning A partition editor software program can be used to create, resize, delete, and manipulate these partitions on the HDD. A partition consists of a range of cylinders of HDD—i.e. each partition is defined by both a start and end cylinder (the size of cylinders varying from disk to disk).

Disk partitioning Benefits of multiple partitions
Separation of the operating system (OS) and program files from user files. This allows image backups (or clones) to be made of only the operating system and installed software.

Having a separate area for operating system virtual memory swapping/paging.

Keeping frequently used programs and data near each other.

Having cache and log files separate from other files. These can change size dynamically and rapidly, potentially making a file system full.

Use of multi-boot setups, which allow users to have more than one operating system on a single computer. For example, one could install Linux, BSD, Mac OS X, Microsoft Windows or other operating systems on different partitions of the same HDD and have a choice of booting into any compatible operating system at power-up.

Protecting or isolating files, to make it easier to recover a corrupted file system or operating system installation. If one partition is corrupted, other file systems may not be affected.

Raising overall computer performance on systems where smaller file systems are more efficient. For instance, large HDDs with only one NTFS file system typically have a very large sequentially accessed Master File Table (MFT) and it generally takes more time to read this MFT than the smaller MFTs of smaller partitions.

This technique, however, is not related to creating multiple partitions, but generally just creating a partition less than the disk size.

For example, a 1 TB disk may have an access time of 12 ms at 200 IOPS (at a limited queue depth) with an average throughput of 100 MB/s. When it is partitioned to 100 GB (and the rest left unallocated) access time may be decreased to 6 ms at 300 IOPS (with a bigger queue depth) with an average throughput of 200 MB/s.

Partitioning for significantly less than the full size available when disk space is not needed can reduce the time for diagnostic tools such as checkdisk to run or for full image backups to run.

Disk partitioning Disadvantages of multiple partitions
Reduces the total space available for user storage on the disk, as it forces the operating system to duplicate certain file system administration areas on the disk for each partition.

Files can still be moved closer to each other on each partition, but those areas themselves will still be far apart on the disk

Increases disk fragmentation because it lowers the average size of continuous free blocks on each partition - as compared to a single partition of the same overall size - after the same amount of data has been written to them.

If the same files on those two partitions would have been stored on a single partition spanning the whole disk, then the 4 GB file could be easily stored in the 6 GB of free space.

Hurts portability and might impose constraints on how entities might be linked together inside the file system

Disk partitioning PC partition types
This section describes the master boot record (MBR) partitioning scheme, as used in DOS, Microsoft Windows and Linux among others on PC compatible computer systems. For examples of partitioning schemes used in other operating systems, see the general article on partition tables.

The total data storage space of a PC HDD can be divided into at most four s, or alternatively three s and an . These partitions are described by 16-byte entries that constitute the Partition Table, located in the master boot record.

The partition type is identified by a 1-byte code found in its partition table entry. Some of these codes (such as 0x05 and 0x0F) may be used to indicate the presence of an extended partition. Most are used by an operating system's bootloader (that examines partition tables) to decide if a partition contains a file system that can be used to mount / access for reading or writing data.

Disk partitioning Primary partition
A primary partition contains one file system

Disk partitioning Primary partition
The partition type code for a primary partition can either correspond to a file system contained within (e.g

Disk partitioning Extended partition
An HDD may contain only one extended partition; the extended partition can be subdivided into multiple logical partitions. In DOS/Windows systems, each logical partition may then be assigned an additional drive letter.

Disk partitioning DOS, Windows, and OS/2
With DOS, Microsoft Windows, and OS/2, a common practice is to use one primary partition for the active file system that will contain the operating system, the page/swap file, all utilities, applications, and user data

Disk partitioning DOS, Windows, and OS/2
Microsoft Windows 2000, XP, Vista, and Windows 7 include a 'Disk Management' program which allows for the creation, deletion and resizing of FAT and NTFS partitions. The Windows Disk Manager in Windows Vista and Windows 7 utilizes a new 1 MB partition alignment scheme which is fundamentally incompatible with Windows 2000, XP, OS/2, DOS as well as many other operating systems.

Disk partitioning Unix-like systems
On Unix-based and Unix-like Operating Systems such as GNU/Linux, OS X, BSD, and Solaris, it is possible to use multiple partitions on a disk device. Each partition can be formatted with a file system or as a swap partition.

Multiple partitions allow directories such as /tmp, /usr, /var, or /home to be allocated their own filesystems. Such a scheme has a number of advantages:

If one file system gets corrupted, the data outside that filesystem/partition may stay intact, minimizing data loss.

Specific file systems can be mounted with different parameters e.g. read-only, or with the execution of setuid files disabled.

A runaway program that uses up all available space on a non-system filesystem does not fill up critical filesystems.

A common default for GNU/Linux desktop systems is to use two partitions: one holding a file system mounted on "/" (the root directory) and a swap partition.

By default, OS X systems also use a single partition for the entire filesystem and use a swap file inside the file system (like Windows) rather than a swap partition.

In Solaris, partitions are sometimes known as slices. This is a conceptual reference to the slicing of a cake into several pieces.

The term "slice" is used in the FreeBSD Operating System to refer to Master Boot Record partitions, to avoid confusion with FreeBSD's own disklabel-based partitioning scheme. However, GUID Partition Table partitions are referred to as "partition" world-wide.

Disk partitioning Multi-boot and mixed-boot systems
Multi-boot systems are computers where the user can boot into one of two or more distinct operating systems (OS) stored in separate storage devices or in separate partitions of the same storage device. In such systems a menu at startup gives a choice of which OS to boot/start (and only one OS at a time is loaded).

Disk partitioning Multi-boot and mixed-boot systems
This is distinct from virtual operating systems, in which one operating system is run as a self-contained virtual "program" within another already-running operating system. (An example is a Windows OS "virtual machine" running from within a Linux OS.)

Disk partitioning GUID Partition Table
The GUID Partition Table (Globally Unique IDentifier) is a part of the Unified Extensible Firmware Interface (UEFI) standard for the layout of the partition table on a physical hard disk. Many operating systems now support this standard.

Disk partitioning Partition recovery
For example, if Windows Disk Management (Windows 2000/XP, etc.) is used to delete a partition, it will overwrite the first sector (relative sector 0) of the partition before removing it

Disk partitioning Compressed disks
Frequently, to avoid confusion, the original partition and the compressed drive had their letters swapped, so that the compressed disk is C:, and the uncompressed area (often containing system files) is given a higher name.

Disk partitioning Compressed disks
Versions of Windows using the NT kernel, including the most recent versions, XP and Vista, contain intrinsic disk compression capability. The use of separate disk compression utilities has declined sharply.

Disk partitioning Further reading
Stéphane Martineau, Jens Olsson, Nick Roberts ( ). The Alt-OS-Development Partition Specification (AODPS). 0.4.

Andries Brouwer (1995–2004). List of partition identifiers for PCs.

Andries Brouwer ( ). Minimal Partition Table Specification.

Disk operating system Such software is referred to as a disk operating system when the storage devices it manages are made of rotating platters, such as floppy disks or hard disks.

Disk operating system In the early days of microcomputers, computer memory space was often limited, so the disk operating system was an extension of the operating system. This component was only loaded if needed. Otherwise, disk access would be limited to low-level operations such as reading and writing disks at the sector-level.

Disk operating system Sometimes, a disk operating system can refer to the entire operating system if it is loaded off a disk and supports the abstraction and management of disk devices. Examples include DOS/360 and FreeDOS. On the PC compatible platform, an entire family of operating systems was called DOS.

Disk operating system History
In the early days of computers, there were no disk drives, floppies or modern flash storage devices

As prices for both disk hardware and operating system software decreased, there were many such microcomputer systems.

Kernal hard disk subsystem for the Commodore 64 and Commodore 128 models stored its DOS on the disk, as is the case with modern systems, and loaded the DOS into RAM at boot time.

In microcomputers, SWTPC's 6800 and 6809 machines used TSC's FLEX disk operating system, Radio Shack's TRS-80 machines used TRS-DOS, their Color Computer used OS-9, and most of the Intel 8080 based machines from IMSAI, MITS (makers of the legendary Altair 8800), Cromemco, North Star, etc., used the CP/M-80 disk operating system

Usually, a disk operating system was loaded from a disk. Only a very few comparable DOSes were stored elsewhere than floppy disks; among these exceptions were the British BBC Micro's optional Disc Filing System, DFS, offered as a kit with a disk controller chip, a ROM chip, and a handful of logic chips, to be installed inside the computer; and Commodore's CBM DOS, located in a ROM chip in each disk drive.

Disk operating system Disk operating systems that were extensions to the OS
The DOS Operating System was the primary Operating System for the Apple Computer's Apple II family of computers, from 1979 with the introduction of the floppy disk drive, until 1983 with the introduction of ProDOS; many people continued using it long after that date. Usually, it was called Apple DOS to distinguish it from MS-DOS.

Commodore DOS, which was used by 8-bit Commodore computers. Unlike most other DOS systems, it was integrated into the disk drives, not loaded into the computer's own memory.

Atari DOS, which was used by the Atari 8-bit family of computers. The Atari OS only offered low-level disk-access, so an extra layer called DOS was booted off a floppy that offered higher level functions such as filesystems.

MSX-DOS, for the MSX computer standard. Initial version, released in 1984, was nothing but MS-DOS 1.0 ported to Z80; but in 1988 it evolved to version 2, offering facilities such as subdirectories, memory management and environment strings. The MSX-DOS kernel resided in ROM (built-in on the disk controller) so basic file access capacity was available even without the command interpreter, by using BASIC extended commands.

Disc Filing System (DFS) This was an optional component for the BBC Micro, offered as a kit with a disk controller chip, a ROM chip, and a handful of logic chips, to be installed inside the computer. See also Advanced Disc Filing System.

GDOS and G+DOS, for the +D and DISCiPLE disk interfaces for the ZX Spectrum.

Disk operating system Disk operating systems that were the main OS
Some disk Operating Systems were the Operating System for the entire computer system.

The DOS/360 initial/simple Operating System for the IBM System/360 family of mainframe computers (it later became DOS/VSE, and was eventually just called VSE).

The DOS Operating System for DEC PDP-11 minicomputers. This OS and the computers it ran on were nearly obsolete by the time PCs became common, with various descendants and other replacements.

Disk image A disk image is a single file or storage device containing the complete contents and structure representing a data storage medium or device, such as a hard drive, tape drive, floppy disk, optical disc, or USB flash drive. A disk image is usually created by creating a complete sector-by-sector copy of the source medium and thereby perfectly replicating the structure and contents of a storage device.

Disk image Some disk imaging utilities omit unused file space from source media, or compress the disk they represent to reduce storage requirements, though these are typically referred to as archive files, as they are not literally disk images.

Disk image Disk image file formats may be open standards, such as the ISO image format for optical disc images, or proprietary to particular software applications.

Disk image A tape image is an image of the contents of a computer's magnetic tape. Image files derived from magnetic tape data storage are known as tape images.

Disk image History Disk images were originally used for backup and disk cloning of floppy disk media, where replication or storage of an exact structure was necessary and efficient.

Disk image Uses Disk images are used heavily for duplication of optical media including DVDs, Blu-ray disks, etc. It is also used to make perfect clones of hard disks.

Disk image Uses Typical uses of virtual drives include the mounting of disk images of CDs and DVDs, and the mounting of virtual hard disks for the purpose of on the fly disk encryption ("OTFE").

Disk image Uses Some operating systems such as Linux and Mac OS X have virtual drive functionality built-in (such as the loop device), while others such as Microsoft Windows require additional software. Windows 8 includes native virtual drive functionality known as Hyper-V.

Disk image Uses Virtual drives are typically read-only, being used to mount existing disk images which are not modifiable by the drive. However some software provides virtual CD/DVD drives which can produce new disk images; this type of virtual drive goes by a variety of names, including "virtual burner".

Disk image Enhancement
Using disk images in a virtual drive allows users to shift data between technologies, for example from CD optical drive to hard disk drive. This may provide advantages such as speed and noise (hard disk drives are typically four or five times faster than optical drives, and also quieter). In addition it may reduce power consumption, since it may allow just one device (a hard disk) to be used instead of two (hard disk plus optical drive).

Disk image Enhancement
Virtual drives may also be used as part of emulation of an entire machine (a virtual machine).

Disk image Software distribution
Since the spread of broadband, CD and DVD images have become a common medium for Linux distributions. Applications for Mac OS X are often delivered online as an Apple Disk Image containing a file system that includes the application, documentation for the application, and so on. Online data and bootable recovery CD images are provided for customers of certain commercial software companies.

Disk images may also be needed to transfer software to machines without a compatible physical disk drive.

For computers running Mac OS X, disk images are the most common file type used for software downloads, typically downloaded with a web browser. The images are typically compressed Apple Disk Image (.dmg suffix) files. They are usually opened by directly mounting them without using a real disk. The advantage compared with some other technologies, such as Zip and RAR archives, is they do not need redundant drive space for the unarchived data.

Software packages for Windows are also sometimes distributed as disk images including ISO images. While Windows versions prior to Windows 7 do not natively support mounting disk images to the files system, several software options are available to do this; see comparison of disc image software.

Disk image Security Virtual hard disks are often used in on the fly disk encryption ("OTFE") software such as FreeOTFE and TrueCrypt, where an encrypted "image" of a disk is stored on the computer. When the disk's password is entered, the disk image is "mounted", and made available as a new volume on the computer. Files written to this virtual drive are written to the encrypted image, and never stored in cleartext.

Disk image Security The process of making a computer disk available for use is called "mounting", the process of removing it is called "dismounting" or "unmounting"; the same terms are used for making an encrypted disk available or unavailable.

Disk image Virtualization
A hard disk image is interpreted by a Virtual Machine Monitor as a system hard disk drive. IT administrators and software developers administer them through offline operations using built-in or third-party tools. In terms of naming, a hard disk image for a certain Virtual Machine monitor has a specific file type extension, e.g., .vmdk for VMware VMDK, .vhd for Xen and Microsoft Hyper-V, .vdi for Oracle VM VirtualBox, etc..

Hard Drive imaging is used in several major application areas:

Forensic imaging or acquisition is the process where the entire drive contents are imaged to a file and checksum values are calculated to verify the integrity (in court cases) of the image file (often referred to as a "hash value"). Forensic images are acquired with the use of software tools. (Some hardware cloning tools have added forensic functionality.)

Drive cloning, as previously mentioned, is typically used to replicate the contents of the Hard Drive for use in another system. This can typically be done by software-only programs as it typically only requires the cloning of file structure and files themselves.

Data recovery imaging (like forensic imaging) is the process of imaging every single sector on the source drive to another medium from which required files can be retrieved

Data recovery imaging must have the ability to pre-configure drives by disabling certain attributes (such as SMART and G-List re-mapping) and the ability to work with unstable drives (drive instability/read instability can be caused by minute mechanical wear and other issues)

Data recovery images may or may not make use of any type of image file. Typically, a data recovery image is performed drive to drive and therefore no image file is required.

There are two schemes predominant across all Virtual Machine Monitor implementations:

Preallocate the entire storage for the virtual disk upon creation

The virtual disk is implemented as either split over a collection of flat files, typically each one is 2GB in size, collectively called a split flat file, or as a single, large monolithic flat file. The pre-allocated storage scheme is also referred to as a thick provisioning scheme.

The virtual disk can again be implemented using split or monolithic files, except that storage is allocated on demand. Several Virtual Machine Monitor implementations initialize the storage with zeros before providing it to the virtual machine that is in operation. The dynamic growth storage scheme is also referred to as a thin provisioning scheme.

There are two modes in which a raw disk can be mapped for use by a virtual machine:

The mapped disk is presented as if it is a logical volume, or a virtual disk file, to the guest Operating System and its real hardware characteristics are hidden. In this mode, file locking provides data protection through isolation for concurrent updates; the copy on write operation enables snapshots. Virtual mode also offers portability across storage hardware because it presents the consistent behavior as a virtual disk file.

In this mode, also called the pass through mode, the Virtual Machine Monitor bypasses the I/O virtualization layer and passes all I/O commands directly to the device. All physical characteristics of the underlying hardware are exposed to the guest Operating System. There is no file locking to provide data protection.

Disk image System backup
Some backup programs only back up user files; boot information and files locked by the operating system, such as those in use at the time of the backup, may not be saved on some operating systems. A disk image contains all files, faithfully replicating all data. For this reason, it is also used for backing up CDs and DVDs.

Creating a bit-identical disk image is one way to ensure the system backup will be exactly as the original

Most commercial imaging software is "user-friendly" and "automatic" but may not create bit-identical images. These programs have most of the same advantages, except that they may allow restoring to partitions of a different size or file-allocation size, and thus may not put files on the same exact sector. Additionally, if they do not support Windows Vista, they may slightly move or realign partitions and thus make Vista unbootable (see Windows Vista startup process).

Disk image Rapid deployment of clone systems
Large enterprises often need to buy or replace new computer systems in large numbers. Installing Operating System and programs into each of them one by one requires a lot of time and effort and has a significant possibility of human error. Therefore, system administrators use disk imaging to quickly clone the fully prepared software environment of a reference system. This method saves time and effort and allows administrators to focus on unique distinctions that each system must bear.

Disk image Rapid deployment of clone systems
There are several types of disk imaging software available that use single instancing technology to reduce the time, bandwidth, and storage required to capture and archive disk images. This makes it possible to rebuild and transfer information-rich disk images at lightning speeds, which is a significant improvement over the days when programmers spent hours configuring each machine within an organization.

Disk image Legacy hardware emulation
Emulators frequently use disk images to simulate the floppy drive of the computer being emulated. This is usually simpler to program than accessing a real floppy drive (particularly if the disks are in a format not supported by the host operating system), and allows a large library of software to be managed.

Disk image Copy protection circumvention
They cannot provide CD or DVD backed data to the computer program such as on-disk image or video files.

Disk image Creation Creating a disk image is achieved with a suitable program. Different disk imaging programs have varying capabilities, and may focus on hard drive imaging (including hard drive backup, restore and rollout), or optical media imaging (CD/DVD images).

Disk image Creation A virtual disk writer or virtual burner is a computer program that emulates an actual disc authoring device such as a CD writer or DVD writer. Instead of writing data to an actual disc, it creates a virtual disk image. A virtual burner, by definition, appears as a disc drive in the system with writing capabilities (as opposed to conventional disc authoring programs that can create virtual disk images), thus allowing software that can burn discs to create virtual discs.

Disk image File formats
In most cases, a file format is tied to a particular software package. The software defines and uses its own, often proprietary, image format, though some formats are widely supported by open standards. These formats are supported by nearly all optical disc software packages.

Disk image Utilities RawWrite and WinImage are examples of floppy disk image file writer/creator for MS-DOS and Microsoft Windows. They can be used to create raw image files from a floppy disk, and write such image files to a floppy.

Disk image Utilities In Unix or similar systems the dd program can be used to create disk images, or to write them to a particular disk. It is also possible to mount and access them at block level using a loop device.

Disk image Utilities Apple Disk Copy can be used on Mac OS systems to create and write disk image files.

Disk image Utilities Authoring software for CDs/DVDs such as Nero Burning ROM can generate and load disk images for optical media.

AmigaOS RAM disk The AmigaOS has a dynamically-sized RAM disk, which resizes itself automatically to its contents. Starting with AmigaOS 2.x, operating System configuration files were loaded into the RAM disk on boot, greatly speeding operating system usage. Other files could be copied to the RAM disk like any standard device for quick modification and retrieval. Also beginning in AmigaOS 2.x, the RAM disk supported file-change notification, which was mostly used to monitor configuration files for changes.

AmigaOS RAM disk Later the ASDG RRD was added to the Fred Fish series of freeware, shareware and public domain software (disks 58 and 241).

AirPort AirPort Disk The AirPort Disk feature shares a hard disk connected to an AirPort Extreme or Time Capsule (though not AirPort Express), as a small-scale NAS. AirPort Disk can be accessed from Windows and Linux as well as Mac OS X using the SMB/CIFS protocol for FAT volumes, and both SMB/CIFS and AFP for HFS+ partitions. NTFS- or exFAT-formatted volumes are not supported.

AirPort AirPort Disk It should be noted that although Windows does not natively support HFS+, an HFS+ volume on an AirPort Disk can be easily accessed from Windows. This is because the SMB/CIFS protocol used to access the disk, and hence access from Windows is filesystem-independent. Therefore HFS+ is a viable option for Windows as well as OS X users, and more flexible than FAT32 as the latter has a 4 GiB file size limit.

AirPort AirPort Disk Recent firmware versions cause the internal disk and any external USB drives to sleep after periods of time as short as 2 minutes.

AirPort AirPort Disk A caveat of the use of AirPort Disk is that the AFP port 548 is reserved for the service, which then does not allow for simultaneous use of port forwarding to provide AFP services to external users. This is also true of a Time Capsule setup for use as a network-based Time Machine Backup location, its main purpose and default configuration. An AirPort administrator must choose between using AirPort Disk and providing remote access to AFP services.

AirPort AirPort Disk The AirPort Extreme or Time Capsule will recognize multiple disks connected via a USB hub. Hard drives over 3 TB are not recognized. Apple's specifications pages do not mention this limitation. As of March 2013, no firmware upgrade is available to remedy this issue.

Operating system - Disk access and file systems
The specific way in which files are stored on a disk is called a file system, and enables files to have names and attributes

Early operating systems generally supported a single type of disk drive and only one kind of file system. Early file systems were limited in their capacity, speed, and in the kinds of file names and directory structures they could use. These limitations often reflected limitations in the operating systems they were designed for, making it very difficult for an operating system to support more than one file system.

While many simpler operating systems support a limited range of options for accessing storage systems, operating systems like UNIX and Linux support a technology known as a virtual file system or VFS

A connected storage device, such as a hard drive, is accessed through a device driver. The device driver understands the specific language of the drive and is able to translate that language into a standard language used by the operating system to access all disk drives. On UNIX, this is the language of block devices.

When the kernel has an appropriate device driver in place, it can then access the contents of the disk drive in raw format, which may contain one or more file systems

Various differences between file systems make supporting all file systems difficult

Operating systems vary on file system support and on the disk formats they may be installed on

Exokernel - Disk storage
The kernel identifies disk blocks to the application program by their physical block address, allowing the application to optimize data placement. When the program initializes its use of the disk, it provides the kernel with a function that the kernel can use to determine which blocks the program controls. The kernel uses this callback to verify that when it allocates a new block, the program claims only the block that was allocated in addition to those it already controlled.

File system - Disk file systems
Some disk file systems are journaling file systems or versioning file systems.

File system - Shared disk file systems
A shared disk file system is one in which a number of machines (usually servers) all have access to the same external disk subsystem (usually a SAN). The file system arbitrates access to that subsystem, preventing write collisions. Examples include GFS2 from Red Hat, GPFS from IBM, SFS from DataPlow, CXFS from SGI and StorNext from Quantum Corporation.

Storage virtualization - Disk management
The software or device providing storage virtualization becomes a common disk manager in the virtualized environment. Logical disks (vdisks) are created by the virtualization software or device and are mapped (made visible) to the required host or server, thus providing a common place or way for managing all volumes in the environment.

Enhanced features are easy to provide in this environment :

Thin Provisioning to maximize storage utilization

This is relatively easy to implement as physical storage is only allocated in the mapping table when it is used.

More physical storage can be allocated by adding to the mapping table (assuming the using system can cope with online expansion)

Similarly disks can be reduced in size by removing some physical storage from the mapping (uses for this are limited as there is no guarantee of what resides on the areas removed)

Digital video recorder - Hard-disk based digital video recorders
Consumer digital video recorders ReplayTV and TiVo were launched at the 1999 Consumer Electronics Show in Las Vegas, USA

Hard-disk based digital video recorders make the "time shifting" feature (traditionally done by a VCR) much more convenient, and also allow for "trick modes" such as pausing live TV, instant replay of interesting scenes, chasing playback where a recording can be viewed before it has been completed, and skipping of advertising. Most DVRs use the MPEG format for compressing the digitized video signals.

Video recording capabilities have become an essential part of the modern set-top box, as TV viewers have wanted to take control of their viewing experiences. As consumers have been able to converge increasing amounts of video content on their set-tops, delivered by traditional ‘broadcast’ cable, satellite and terrestrial] as well as IP networks, the ability to capture programming and view it whenever they want has become a must-have function for many consumers.

At the 1999 CES, Dish Network demonstrated the hardware that would later have DVR capability with the assistance of Microsoft software. which also included WebTV Networks internet TV. By the end of 1999 the Dishplayer had full DVR capabilities and within a year, over 200,000 units were sold.

In the UK, digital video recorders are often referred to as "plus boxes" (such as BSKYB's Sky+ and Virgin Media's V+ which integrates an HD capability, and the subscription free Freesat+ and Freeview+)

Many satellite, cable and IPTV companies are incorporating digital video recording functions into their set-top box, such as with DirecTiVo, DISHPlayer/DishDVR, Scientific Atlanta Explorer 8xxx from Time Warner, Total Home DVR from AT&T U-verse, Motorola 6412 from Comcast and others, Moxi Media Center by Digeo (available through Charter, Adelphia, Sunflower, Bend Broadband, and soon Comcast and other cable companies), or Sky+

The digital video recorder simply stores the digital stream directly to disk

In the United States, the FCC has ruled that starting on July 1, 2007, consumers will be able to purchase a set-top box from a third-party company, rather than being forced to purchase or rent the set-top box from their cable company

Sony - Disk storage In 1983 Sony introduced 90 mm micro diskettes (better known as 3.5-inch (89 mm) floppy disks), which it had developed at a time when there were 4" floppy disks, and a lot of variations from different companies, to replace the then on-going 5.25" floppy disks. Sony had great success and the format became dominant. 3.5" floppy disks gradually became obsolete as they were replaced by current media formats.

RAID - Scenarios other than disk failure
While RAID may protect against physical drive failure, the data are still exposed to operator, software, hardware, and virus destruction. Many studies cite operator fault as the most common source of malfunction, such as a server operator replacing the incorrect drive in a faulty RAID, and disabling the system (even temporarily) in the process.

Disk mirroring In data storage, disk mirroring is the replication of logical disk volumes onto separate physical hard disks in real time to ensure continuous availability. It is most commonly used in RAID 1. A mirrored volume is a complete logical representation of separate volume copies.

Disk mirroring In a Disaster Recovery context, mirroring data over long distance is referred to as storage replication. Depending on the technologies used, replication can be performed synchronously, asynchronously, semi-synchronously, or point-in-time. Replication is enabled via microcode on the disk array controller or via server software. It is typically a proprietary solution, not compatible between various storage vendors.

Disk mirroring Mirroring is typically only synchronous. Synchronous writing typically achieves a Recovery Point Objective (RPO) of zero lost data. Asynchronous replication can achieve an RPO of just a few seconds while the remaining methodologies provide an RPO of a few minutes to perhaps several hours.

Disk mirroring - Explanation
Mirroring may be done locally where it is specifically to cater for disk unreliability, or it may be done remotely where it forms part of a more sophisticated disaster recovery scheme, or it may be done both locally and remotely, especially for high availability systems

In less sophisticated systems, the system is operated on the remaining disk until such time as a spare disk can be installed with minimum disruption.

The copying of data from one pair of a mirror to another is sometimes called resilvering though more commonly it is simply known as rebuilding.

Mirroring can be performed site to site either by rapid data links, for example fibre optic links, which over distances of 500 m or so can maintain adequate performance to support real-time mirroring. Longer distances or slower links maintain mirrors using an asynchronous copying system. For remote disaster recovery systems, this mirroring may not be done by integrated systems but simply by additional applications on master and slave machines.

Disk mirroring - Other benefits of mirroring
This is an important consideration in hardware configurations that frequently access the data on the disk.

Disk mirroring - Other benefits of mirroring
In some implementations, the mirrored disk can be split off and used for data backup, allowing the first disk to remain active. However merging the two disks then may require a synchronization period if any write I/O activity has occurred to the mirrored disk.

Disk mirroring - Other schemes
Some mirroring schemes employ three disks, with two of the disks for the redundancy mirroring and the third to be split off for performing backups. In EMC nomenclature, these third disks are called Business Continuance Volumes (BCVs).

Hard disk drive An HDD consists of one or more rigid ("hard") rapidly rotating disks (platters) with magnetic heads arranged on a moving actuator arm to read and write data to the surfaces.

Hard disk drive Introduced by IBM in 1956, HDDs became the dominant secondary storage device for general purpose computers by the early 1960s

The primary characteristics of an HDD are its capacity and performance
Hard disk drive The primary characteristics of an HDD are its capacity and performance

Hard disk drive The two most common form factors for modern HDDs are 3.5-inch in desktop computers and 2.5-inch in laptops. HDDs are connected to systems by standard interface cables such as SATA (Serial ATA), USB or SAS (Serial attached SCSI) cables.

Hard disk drive As of 2012, the primary competing technology for secondary storage is flash memory in the form of solid-state drives (SSDs). HDDs are expected to remain the dominant medium for secondary storage due to predicted continuing advantages in recording capacity and price per unit of storage; but SSDs are replacing HDDs where speed, power consumption and durability are more important considerations than price and capacity.

Hard disk drive - History
HDDs were introduced in 1956 as data storage for an IBM real-time transaction processing computer and were developed for use with general purpose mainframe and minicomputers. The first IBM drive, the 350 RAMAC, was approximately the size of two refrigerators and stored 5 million 6-bit characters (the equivalent of 3.75 million 8-bit bytes or 3.75 MB or megabytes) on a stack of 50 disks.

In 1961 IBM introduced the model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk pack. Users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s. Non-removable HDDs were called fixed disk drives.

Some high performance HDDs were manufactured with one head per track, e.g., IBM 2305 so that no time was lost physically moving the heads to a track. Known as Fixed-Head or Head-Per-Track disk drives they were very expensive and are no longer in production.

Instead, the first models of "Winchester technology" drives featured a removable disk module, which included both the disk pack and the head assembly, leaving the actuator motor in the drive upon removal

Drives with non-removable eight-inch platters appeared, and then drives that used a 5 1⁄4 in (130 mm) form factor (a mounting width equivalent to that used by contemporary floppy disk drives)

As the 1980s began, HDDs were a rare and very expensive additional feature on PCs; however by the late 1980s, their cost had been reduced to the point where they were standard on all but the cheapest PC.

Most HDDs in the early 1980s were sold to PC end users as an external, add-on subsystem. The subsystem was not sold under the drive manufacturer's name but under the subsystem manufacturer's name such as Corvus Systems and Tallgrass Technologies, or under the PC system manufacturer's name such as the Apple ProFile. The IBM PC/XT in 1983 included an internal 10 MB HDD, and soon thereafter internal HDDs proliferated on personal computers.

External HDDs remained popular for much longer on the Apple Macintosh. Every Mac made between 1986 and 1998 has a SCSI port on the back, making external expansion easy; also, "toaster" Compact Macs did not have easily accessible HDD bays (or, in the case of the Mac Plus, any hard drive bay at all), so on those models, external SCSI disks were the only reasonable option.

Driven by areal density doubling every two to four years since their invention (an observation known as Kryder's law, similar to Moore's Law), HDDs have continuously improved their characteristics; a few highlights include:

Physical volume of HDD decreasing from 68 cubic feet (1.9 m3) (comparable to a large side-by-side refrigerator), to less than 20 millilitres (0.70 imp fl oz; 0.68 US fl oz), a 100,000-to-1 decrease.

Weight decreasing from 2,000 pounds (910 kg) to 48 grams (1.7 oz), a 20,000-to-1 decrease.

Price decreasing from about US$15,000 per megabyte to less than $ per megabyte ($90/1.5 terabyte), a greater than 250-million-to-1 decrease.

Market application expanding from mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content.

Hard disk drive - Magnetic recording
An HDD records data by magnetizing a thin film of ferromagnetic material[note 3] on a disk. Sequential changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization. User data is encoded using an encoding scheme, such as run-length limited encoding,[note 4] which determines how the data is represented by the magnetic transitions.

A typical HDD design consists of a spindle that holds flat circular disks, also called platters, which hold the recorded data. The platters are made from a non-magnetic material, usually aluminium alloy, glass, or ceramic, and are coated with a shallow layer of magnetic material typically 10–20 nm in depth, with an outer layer of carbon for protection. For reference, a standard piece of copy paper is 0.07–0.18 millimetre (70,000–180,000 nm).

The platters in contemporary HDDs are spun at speeds varying from 4,200 rpm in energy-efficient portable devices, to 15,000 rpm for high performance servers. The first HDDs spun at 1,200 rpm and, for many years, 3,600 rpm was the norm. Today, the platters in most consumer HDDs spin in the range of 5,400 rpm to 7,200 rpm.

Information is written to and read from a platter as it rotates past devices called read-and-write heads that operate very close (often tens of nanometers) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it.

Early hard disk drives wrote data at some constant bits per second, resulting in all tracks having the same amount of data per track but modern drives (since the 1990s) use zone bit recording—increasing the write speed from inner to outer zone and thereby storing more data per track in the outer zones.

In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of thermal effects

Hard disk drive - Components
The disk motor has an external rotor attached to the disks; the stator windings are fixed in place

The actuator is a permanent magnet and moving coil motor that swings the heads to the desired position. A metal plate supports a squat neodymium-iron-boron (NIB) high-flux magnet. Beneath this plate is the moving coil, often referred to as the voice coil by analogy to the coil in loudspeakers, which is attached to the actuator hub, and beneath that is a second NIB magnet, mounted on the bottom plate of the motor (some drives only have one magnet).

The voice coil itself is shaped rather like an arrowhead, and made of doubly coated copper magnet wire

Modern disk firmware is capable of scheduling reads and writes efficiently on the platter surfaces and remapping sectors of the media which have failed.

Hard disk drive - Error rates and handling
For example, a typical 1 TB hard disk with 512-byte sectors, also provides additional capacity of about 93 GB for the ECC data.

In the newest drives, as of 2009, low-density parity-check codes (LDPC) were supplanting Reed-Solomon; LDPC codes enable performance close to the Shannon Limit and thus provide the highest storage density available.

Typical HDDs attempt to "remap" the data in a physical sector that is failing to a spare physical sector — hopefully while the errors in the bad sector are still few enough that the ECC can recover the data without loss. The S.M.A.R.T-Self-Monitoring, Analysis and Reporting Technology system counts the total number of errors in the entire HDD fixed by ECC and the total number of remappings, as the occurrence of many such errors may predict HDD failure.

An enterprise disk with a Fibre Channel interface, which uses 520 byte sectors to support the Data Integrity Field standard to combat data corruption, specifies similar error rates in 2005.

Hard disk drive - Future development
HDD areal densities have shown a long term compound annual growth rate not substantively different from Moore's Law, most recently in the range of 20-25% annually, with desktop 3.5" drives estimated to hit 12 TB around New magnetic storage technologies are being developed to support higher areal density growth and maintain the competitiveness of HDDs with potentially competitive products such as flash memory-based solid-state drives (SSDs). These new HDD technologies include:

Current Perpendicular to Plane giant magnetoresistance (CPP/GMR) heads

With these new technologies the relative position of HDDs and SSDs with regard to their cost and performance is not projected to change through 2016.

Hard disk drive - Capacity
The capacity of an HDD reported to an end user by the operating system is less than the amount stated by a drive or system manufacturer due to amongst other things, different units of measuring capacity, capacity consumed by the file system and/or redundancy.

Hard disk drive - Calculation
Because modern disk drives appear to their interface as a contiguous set of logical blocks their gross capacity can be calculated by multiplying the number of blocks by the size of the block. This information is available from the manufacturer's specification and from the drive itself through use of special utilities invoking low level commands.

The gross capacity of older HDDs can be calculated by multiplying for each zone of the drive the number of cylinders by the number of heads by the number of sectors/zone by the number of bytes/sector (most commonly 512) and then summing the totals for all zones. Some modern SATA drives will also report cylinder-head-sector (C/H/S) values to the CPU but they are no longer actual physical parameters since the reported numbers are constrained by historic operating-system interfaces.

The old C/H/S scheme has been replaced by logical block addressing. In some cases, to try to "force-fit" the C/H/S scheme to large-capacity drives, the number of heads was given as 64, although no modern drive has anywhere near 32 platters: the typical 2 TB hard disk as of 2013 has two 1 TB platters (and 4 TB drives use four platters).

Hard disk drive - Redundancy
In modern HDDs, spare capacity for defect management is not included in the published capacity; however in many early HDDs a certain number of sectors were reserved for spares, thereby reducing capacity available to end users.

In some systems, there may be hidden partitions used for system recovery that reduce the capacity available to the end user.

For RAID subsystems, data integrity and fault-tolerance requirements also reduce the realized capacity

Hard disk drive - File system use
The presentation of an HDD to its host is determined by its controller. This may differ substantially from the drive's native interface particularly in mainframes or servers.

Modern HDDs, such as SAS and SATA drives, appear at their interfaces as a contiguous set of logical blocks; typically 512 bytes long but the industry is in the process of changing to 4,096-byte logical blocks; see Advanced Format.

The process of initializing these logical blocks on the physical disk platters is called low level formatting which is usually performed at the factory and is not normally changed in the field.[note 5]

Examples of data structures stored on disk to retrieve files include the file allocation table (FAT) in the MS-DOS file system and inodes in many UNIX file systems, as well as other operating system data structures

Hard disk drive - Units Advertised capacity by manufacturer (using decimal multiples) Expected capacity by consumers in class action (using binary multiples) Reported capacity

Hard disk drive - Units Windows (using binary multiples) Mac OS X (using decimal multiples)

Hard disk drive - Units The total capacity of HDDs is given by manufacturers in megabytes (1 MB = 1,000,000 bytes), gigabytes (1 GB = 1,000,000,000 bytes) or terabytes (1 TB = 1,000,000,000,000 bytes). This numbering convention, where prefixes like mega- and giga- denote powers of 1,000, is also used for data transmission rates and DVD capacities. However, the convention is different from that used by manufacturers of memory (RAM, ROM) and CDs, where prefixes like kilo- and mega- mean powers of 1,024.

Hard disk drive - Units The practice of using prefixes assigned to powers of 1,000 within the HDD and computer industries dates back to the early days of computing. By the 1970s million, mega and M were consistently being used in the powers of 1,000 sense to describe HDD capacity.

Hard disk drive - Units Computers do not internally represent HDD or memory capacity in powers of 1,024; reporting it in this manner is just a convention. Microsoft Windows uses the powers of 1,024 convention when reporting HDD capacity, thus an HDD offered by its manufacturer as a 1 TB drive is reported by these OSes as a 931 GB HDD. Mac OS X 10.6 ("Snow Leopard"), uses powers of 1,000 when reporting HDD capacity.

Hard disk drive - Units In the case of "mega-", there is a nearly 5% difference between the powers of 1,000 definition and the powers of 1,024 definition

Hard disk drive - Units In December 1998, standards organizations addressed these dual definitions of the conventional prefixes by standardizing on unique binary prefixes and prefix symbols to denote multiples of 1,024, such as "mebibyte (MiB)", which exclusively denotes 220 or 1,048,576 bytes. This standard has seen little adoption by the computer industry, and the conventionally prefixed forms of "byte" continue to denote slightly different values depending on context.

Hard disk drive - Form factors
Mainframe and minicomputer hard disks were of widely varying dimensions, typically in free standing cabinets the size of washing machines or designed to fit a 19" rack. In 1962, IBM introduced its model 1311 disk, which used 14 inch (nominal size) platters. This became a standard size for mainframe and minicomputer drives for many years. Such large platters were never used with microprocessor-based systems.

With increasing sales of microcomputers having built in floppy-disk drives (FDDs), HDDs that would fit to the FDD mountings became desirable. Thus HDD Form factors, initially followed those of 8-inch, 5.25-inch, and 3.5-inch floppy disk drives. Because there were no smaller floppy disk drives, smaller HDD form factors developed from product offerings or industry standards.

9.5 in × in × in (241.3 mm × mm × 362 mm). In 1979, Shugart Associates' SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8" FDD.

Most desktop models of drives for optical 120 mm disks (DVD, CD) use the half height 5¼" dimension, but it fell out of fashion for HDDs

4 in × 1 in × 5.75 in (101.6 mm × 25.4 mm × 146 mm) = cm³. This smaller form factor is similar to that used in an HDD by Rodime in 1983, which was the same size as the "half height" 3½" FDD, i.e., 1.63 inches high. Today, the 1-inch high ("slimline" or "low-profile") version of this form factor is the most popular form used in most desktops.

2.75 in × 0.275–0.59 in × in (69.85 mm × 7–15 mm × 100 mm) = – cm3

54 mm × 8 mm × 71 mm = cm³. This form factor, originally introduced by Integral Peripherals in 1993, evolved into the ATA-7 LIF with dimensions as stated. For a time it was increasingly used in digital audio players and subnotebooks, but its popularity decreased to the point where this form factor is increasingly rare and only a small percentage of the overall market.

42.8 mm × 5 mm × 36.4 mm. This form factor was introduced in 1999 as IBM's Microdrive to fit inside a CF Type II slot. Samsung calls the same form factor "1.3 inch" drive in its product literature.

24 mm × 5 mm × 32 mm. Toshiba announced this form factor in January 2004 for use in mobile phones and similar applications, including SD/MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba manufactured a 4 GB (MK4001MTD) and an 8 GB (MK8003MTD) version and holds the Guinness World Record for the smallest HDD.

As of 2012, 2.5-inch and 3.5-inch hard disks were the most popular sizes.

By 2009 all manufacturers had discontinued the development of new products for the 1.3-inch, 1-inch and 0.85-inch form factors due to falling prices of flash memory, which has no moving parts.

While these sizes are customarily described by an approximately correct figure in inches, actual sizes have long been specified in millimeters.

Hard disk drive - Time to access data
Delay may also occur if the drive disks are stopped to save energy.

An HDD's Average Access Time is its average Seek time which technically is the time to do all possible seeks divided by the number of all possible seeks, but in practice is determined by statistical methods or simply approximated as the time of a seek over one-third of the number of tracks.

Defragmentation is a procedure used to minimize delay in retrieving data by moving related items to physically proximate areas on the disk. Some computer operating systems perform defragmentation automatically. Although automatic defragmentation is intended to reduce access delays, performance will be temporarily reduced while the procedure is in progress.

Time to access data can be improved by increasing rotational speed (thus reducing latency) and/or by reducing the time spent seeking

Hard disk drive - Seek time
Average seek time ranges from 3 ms for high-end server drives, to 15 ms for mobile drives, with the most common mobile drives at about 12 ms and the most common desktop type typically being around 9 ms

Hard disk drive - Seek time
Some desktop and laptop computer systems allow the user to make a tradeoff between seek performance and drive noise

Hard disk drive - Latency
Latency is the delay for the rotation of the disk to bring the required disk sector under the read-write mechanism. It depends on rotational speed of a disk, measured in revolutions per minute (rpm). Average rotational latency is shown in the table below, based on the statistical relation that the average latency in milliseconds for such a drive is one-half the rotational period.

Hard disk drive - Data transfer rate
Data transfer rate (read/write) can be measured by writing a large file to disk using special file generator tools, then reading back the file

Hard disk drive - Data transfer rate
While areal density advances by increasing both the number of tracks across the disk and the number of sectors per track, only the latter increases the data transfer rate for a given rpm

Hard disk drive - Other considerations
Other performance considerations include power consumption, audible noise, and shock resistance.

Hard disk drive - Access and interfaces
HDDs are accessed over one of a number of bus types, including as of 2011 parallel ATA (PATA, also called IDE or EIDE; described before the introduction of SATA as ATA), Serial ATA (SATA), SCSI, Serial Attached SCSI (SAS), and Fibre Channel. Bridge circuitry is sometimes used to connect HDDs to buses with which they cannot communicate natively, such as IEEE 1394, USB and SCSI.

Modern HDDs present a consistent interface to the rest of the computer, no matter what data encoding scheme is used internally

Modern interfaces connect an HDD to a host bus interface adapter (today typically integrated into the "south bridge") with one data/control cable. Each drive also has an additional power cable, usually direct to the power supply unit.

Small Computer System Interface (SCSI), originally named SASI for Shugart Associates System Interface, was standard on servers, workstations, Commodore Amiga, Atari ST and Apple Macintosh computers through the mid-1990s, by which time most models had been transitioned to IDE (and later, SATA) family disks. The range limitations of the data cable allows for external SCSI devices.

Integrated Drive Electronics (IDE), later standardized under the name AT Attachment (ATA, with the alias P-ATA or PATA (Parallel ATA) retroactively added upon introduction of SATA) moved the HDD controller from the interface card to the disk drive

EIDE was an unofficial update (by Western Digital) to the original IDE standard, with the key improvement being the use of direct memory access (DMA) to transfer data between the disk and the computer without the involvement of the CPU, an improvement later adopted by the official ATA standards. By directly transferring data between memory and disk, DMA eliminates the need for the CPU to copy byte per byte, therefore allowing it to process other tasks while the data transfer occurs.

The latter are traditionally reserved for larger devices, such as servers or disk array controllers.

Serial Attached SCSI (SAS). The SAS is a new generation serial communication protocol for devices designed to allow for much higher speed data transfers and is compatible with SATA. SAS uses a mechanically identical data and power connector to standard 3.5-inch SATA1/SATA2 HDDs, and many server-oriented SAS RAID controllers are also capable of addressing SATA HDDs. SAS uses serial communication instead of the parallel method found in traditional SCSI devices but still uses SCSI commands.

Serial ATA (SATA). The SATA data cable has one data pair for differential transmission of data to the device, and one pair for differential receiving from the device, just like EIA-422. That requires that data be transmitted serially. A similar differential signaling system is used in RS485, LocalTalk, USB, FireWire, and differential SCSI.

Hard disk drive - Integrity and failure
Due to the extremely close spacing between the heads and the disk surface, HDDs are vulnerable to being damaged by a head crash—a failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, contamination of the drive's internal enclosure, wear and tear, corrosion, or poorly manufactured platters and heads.

Breather holes can be seen on all disk drives—they usually have a sticker next to them, warning the user not to cover the holes

For giant magnetoresistive (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) still results in the head temporarily overheating, due to friction with the disk surface, and can render the data unreadable for a short period until the head temperature stabilizes (so called "thermal asperity", a problem which can partially be dealt with by proper electronic filtering of the read signal).

In the event of disk-platter failures, disassembly and imaging of the disk platters may be required

A 2011 summary of research into SSD and magnetic disk failure patterns by Tom's Hardware summarized research findings as follows:

MTBF does not indicate reliability; the annualized failure rate is higher and usually more relevant.

Magnetic disks do not have a specific tendency to fail during early use, and temperature only has a minor effect; instead, failure rates steadily increase with age.

S.M.A.R.T. warns of mechanical issues but not other issues affecting reliability, and is therefore not a reliable indicator of condition.

Failure rates of drives sold as "enterprise" and "consumer" are "very much similar", although customized for their different environments.

In drive arrays, one drive's failure significantly increases the short-term chance of a second drive failing.

Hard disk drive - External removable drives
External removable HDDs[note 10] typically connect via USB. Plug and play drive functionality offers system compatibility and features large storage options and portable design. External HDDs are available in 2.5" and 3.5" sizes, and as of March 2012 their capacities generally range from 160 GB to 2 TB. Common sizes are 160 GB, 250 GB, 320 GB, 500 GB, 640 GB, 750 GB, 1 TB, 2 TB, 3 TB and 4 TB

External HDDs are available as preassembled integrated products or may be assembled by combining an external enclosure (with USB or other interface) with a separately purchased drive.

Features such as biometric security or multiple interfaces are available at a higher cost.

External hard drives generally have a slower transfer rate than that of an internally mounted hard drive connecting through SATA.

There are pre-assembled external hard disk drives that, when taken out from its enclosure, cannot be used internally on a laptop or desktop due to the embedded interface on its printed circuit board.

Hard disk drive - Market segments
Desktop HDDs typically store between 60 GB and 4 TB and rotate at 5,400 to 10,000 rpm, and have a media transfer rate of 0.5 Gbit/s or higher (1 GB = 109 bytes; 1 Gbit/s = 109 bit/s). As of September 2011, the highest capacity consumer HDDs store 4 TB.

Mobile HDDs or laptop HDDs, smaller than their desktop and enterprise counterparts, tend to be slower and have lower capacity. Mobile HDDs spin at 4,200 rpm, 5,200 rpm, 5,400 rpm, or 7,200 rpm, with 5,400 rpm being typical. 7,200 rpm drives tend to be more expensive and have smaller capacities, while 4,200 rpm models usually have very high storage capacities. Because of smaller platter(s), mobile HDDs generally have lower capacity than their greater desktop counterparts.

Enterprise HDDs are typically used with multiple-user computers running enterprise software

Consumer electronics HDDs include drives embedded into digital video recorders and automotive vehicles. The former are configured to provide a guaranteed streaming capacity, even in the face of read and write errors, while the latter are built to resist larger amounts of shock.

Hard disk drive - Manufacturers and sales
More than 200 companies have manufactured HDDs over time. But consolidations have concentrated production into just three manufacturers today: Western Digital, Seagate, and Toshiba.

Hard disk drive - Manufacturers and sales
Worldwide revenues for HDDs shipments are expected to reach $33 billion in 2013, down about 12% from $37.8 billion in This corresponds to a 2013 unit shipment forecast of 552 million compared to 577 million units in 2012 and 624 million units in The estimated 2013 market shares are about 40-45% each for Seagate and Western Digital and 13-16% for Toshiba

Hard disk drive - Icons HDDs are traditionally symbolized as a stylized stack of platters or as a cylinder and are found in diagrams, or on lights to indicate HDD access. In most modern operating systems, HDDs are represented by an illustration or photograph of the drive enclosure.

Hard disk drive - Notes Jump up ^ This is the original filing date of the application which led to US Patent 3,503,060, generally accepted as the definitive disk drive patent.

Hard disk drive - Notes Jump up ^ Further terms used to describe hard disk drives include hard drive, hard disk, disk drive, disk file, direct access storage device (DASD), fixed disk, CKD disk, and Winchester disk drive (after the IBM 3340). The term DASD includes other devices besides disks.

Hard disk drive - Notes Jump up ^ Initially gamma iron oxide particles in an epoxy binder, the recording layer in a modern HDD typically is domains of a granular Cobalt-Chrome-Platinum based alloy physically isolated by an oxide to enable perpendicular recording.

Hard disk drive - Notes Jump up ^ Historically a variety of run-length limited codes have been used in magnetic recording including for example, codes named FM, MFM and GCR which are no longer used in modern HDDs.

Hard disk drive - Notes Jump up ^ See: Low-Level Formatting. However, some enterprise SAS drives have other block sizes such as 520, 524 and 528 bytes which can be changed in the field.

Jump up ^ 320 GB for IDE-based barebone.
Hard disk drive - Notes Jump up ^ 320 GB for IDE-based barebone.

Jump up ^ 240 GB for IDE-based barebone.
Hard disk drive - Notes Jump up ^ 240 GB for IDE-based barebone.

Hard disk drive - Notes Jump up ^ The Quantum Bigfoot TS used a maximum of 3 platters, other earlier and lower capacity product used up to 4 platters in a 5.25" HH form factor, e.g., Microscience HH1090 circa 1989.

Hard disk drive - Notes Jump up ^ These differ from removable disk media, e.g., disk packs or data modules, in that they include, for example, actuators, drive electronics, motors.

Hard disk drive - Further reading
Mueller, Scott (2011). Upgrading and Repairing PCs (20th ed.). Que. ISBN

Personal computer - Hard disk
The use of a sealed assembly allowed the use of positive air pressure to drive out particles from the surface of the disk, which improves reliability.

If the mass storage controller provides for expandability, a PC may also be upgraded by the addition of extra hard disk or optical disc drives. For example, BD-ROMs, DVD-RWs, and various optical disc recorders may all be added by the user to certain PCs. Standard internal storage device connection interfaces are PATA, Serial ATA, and SCSI.

Solid state drives or SSD has begun replacing traditional mechanical hard drives.

Disk enclosure Factory-assembled external hard disk drives, external DVD-ROM drives, and others consist of a storage device in a disk enclosures.

Disk enclosure - Benefits
Key benefits to using external disk enclosures include:

Adding additional storage space and media types to small form factor and laptop computers, as well as sealed embedded systems such as digital video recorders and Video Game consoles.

Adding RAID capabilities to computers that lack RAID controllers or adequate space for additional drives.

Transferring data between non-networked computers, humorously known as sneakernet.

Adding an easily removable backup source with a separate power supply from the connected computer.

Using a network-attached storage-capable enclosure over a network to share data or provide a cheap off-site backup solution.

Preventing the heat from a disk drive from increasing the heat inside an operating computer case.

Recovering the data from a damaged computer's hard drive, particularly when it doesn't share the same interface with the computer used to perform the recovery.

Lower the cost of removable storage by reusing hardware designed for internal use.

Disk enclosure - Consumer enclosures
In the consumer market, commonly used configurations of drive enclosures utilize magnetic hard drives or optical disc drives inside USB, FireWire, or Serial ATA enclosures. External 3.5-in floppy drives are also fairly common, following a trend to not integrate floppy drives into compact and laptop computers. Pre-built external drives are available through all major manufacturers of hard drives, as well as several third-parties.

These may also be referred to as a caddy – a sheath, typically plastic or metallic, within which a hard disk drive can be placed and connected with the same type of adapters as a conventional motherboard and power supply would use. The exterior of the caddy typically has two female sockets, used for data transfer and power.

some larger caddies can support several devices at once and can feature either separate outputs to connect each device to a different computer, or a single output to connect both over the same data cable

certain caddes don't require a power supply, instead depending for power on the computer to which they are connected

some caddies have integrated fans with which to keep the drives within at a cool temperature

caddies for all major standards exist, supporting for example ATA, SCSI and SATA drives and USB, SCSI and FireWire outputs

relatively high transfer speed; typically faster than other common portable media such as CDs, DVDs and USB flash drives, slower than drives connected using solely ATA, SCSI and SATA connectors

storage; typically larger than CDs, DVDs and USB flash drives

price-to-storage ratio; typically better than CDs, DVDs and USB flash drives

power; most variants require a supply, unlike CDs, DVDs and USB flash drives...

Disk enclosure - Form factors
Multiple drives: RAID-enabled enclosures and iSCSI enclosures commonly hold multiple drives. High-end and server-oriented chassis are often built around 3.5-in drives in hot-swappable drive caddies.

Most desktop models of drives for optical 120-mm disks (DVD-ROM or CD-ROM drives, CD or DVD burners), are designed to be mounted into a so-called "5.25-inch slot", which obtained its nickname because this slot size was initially used by drives for 5.25-inch-diameter (133 mm) floppy disks in the IBM PC AT

This smaller, 4-inch-wide (100 mm) disk-drive form factor was introduced with the Apple Macintosh series in 1984, and later adopted throughout the industry beginning widely with the IBM PS/2 series in 1987, which included drives of this size for 90-mm ("3.5-inch") floppy disks. This form factor is today used by most desktop hard drives. They usually have 10 mounting holes with American 6-32 UNC 2B threads: three on each side and four on the bottom.

This even smaller, 2.75-inch-wide (70 mm) form factor is widely used today in notebook computers and similar small-footprint devices. One commonplace feature for these drives is radically lower power consumption than is found in larger drives. This enables enclosure vendors to power the devices directly from the host device's USB or other external bus, in most cases.

"1.8 inch" drive: Found in extremely compact devices, such as certain portable media players and smaller notebooks, these devices are not standardized like their 2.5 inch cousins.

A range of other form factors has emerged for mobile devices

While they are less common now than they once were, it is also possible to purchase a drive chassis and mount that will convert a 3.5-in hard drive into a removable hard disk that can be plugged into and removed from a mounting bracket permanently installed in a desktop PC case. The mounting bracket carries the data bus and power connections over a proprietary connector, and converts back into the drive's native data bus format and power connections inside the drive's chassis.

Disk enclosure - Enterprise enclosures
In enterprise storage the term refers to a larger physical chassis. The term can be used both in reference to network-attached storage (NAS) and components of a storage area network (SAN) or be used to describe a chassis directly attached to one or more servers over an external bus. Like their conventional server brethren, these devices may include a backplane, temperature sensors, cooling systems, enclosure management devices, and redundant power supplies.

Disk enclosure - Native drive interfaces
SCSI, SAS, Fibre Channel, eSATAp, and eSATA interfaces can be used to directly connect the external hard drive to an internal host adapter, without the need for any intervening controller

Disk enclosure - Direct attach serial interfaces
USB or FireWire connections are typically used to attach consumer class external hard drives to a computer. Unlike SCSI, eSATA, or SAS these require circuitry to convert the hard disk's native signal to the appropriate protocol. Parallel ATA and internal Serial ATA hard disks are frequently connected to such chassis because nearly all computers on the market today have USB or FireWire ports, and these chassis are inexpensive and easy to find.

Disk enclosure - Network protocols
A newer technology NAS, has been applied to some disk enclosures, which allows network ability, direct connection (e.g., USB) and even RAID features.

Random-access memory - RAM disk
Software can "partition" a portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a RAM disk. A RAM disk loses the stored data when the computer is shut down, unless memory is arranged to have a standby battery source.

MobileMe - iDisk Another way to use iDisk to share several files easily was by placing them in the iDisk Public Folder, which could also be password protected.

Macintosh hardware - Disks
The earliest Macintoshes used a proprietary serial port (a 19-pin D-subminiature connector) for external floppy or hard drives, until SCSI was introduced with the Macintosh Plus. SCSI remained the Macintosh drive medium of choice until the mid-1990s, when less expensive ATA drives were introduced, first on budget models, then across the whole range. Current Macs use Serial ATA for internal hard drives and optical drives, and FireWire or USB 2.0 for external drives.

Only the Power Macs, Mac Pros, Xserves, and MacBook have user-accessible drive bays to allow one or more hard drives to be installed internally. All other machines have one dedicated space for one hard drive.

All Macs have one optical drive, except the Mac Pro, which can optionally include two.

Mac OS X, understands the Mac OS Standard and Mac OS Extended file systems. It is also capable of using disks formatted with Windows's FAT or NTFS file systems, as well as the Unix File System. Currently, Mac OS X Leopard betas have read-only support for ZFS, while paid members of Apple Developer Connection get access to an in-development read-write ZFS driver.

Macintosh External Disk Drive
Apple produced only one external 3.5" drive exclusively for use with the Apple II series called the Apple UniDisk 3.5.

Macintosh External Disk Drive - 400K
Therefore the external floppy disk drive became critical to the success of the Macintosh.

Macintosh External Disk Drive - 800K
However, it was clear that the Mac's original MFS disk formatting system would be inefficient for such a large drive and Apple quickly began adopting for the Mac the hierarchical based SOS filing system introduced with the Apple III and long since implemented in ProDOS for the Apple II series & the Lisa

Macintosh External Disk Drive - Apple UniDisk 3.5
The 3.5" format was not very popular in the Apple II community (excluding the 16-bit Apple IIGS) as most software was released in the 5.25" format to accommodate the existing installed Disk II drives.

Macintosh External Disk Drive - Macintosh 800K External Drive
Designed primarily to run on Macs with the new 128K ROM which contained the necessary code to support the drive, it could be used with Macs with older 64K ROMs if the proper software was loaded from the system folder of a Hard Disk 20 into the Mac's RAM

Macintosh External Disk Drive - Apple 3.5" Drive
Designed as a universal external drive replacement, the Apple 3.5" Drive was eventually compatible with the remaining Apple II models in production upon the introduction of the Apple IIc Plus and the Apple II 3.5 Disk Controller Card for the Apple IIe.

Macintosh External Disk Drive - 1.44MB
In Apple's pursuit of cross-compatibility with DOS & Windows based systems to attract more business customers, they adopted the new format, thus confirming it as the first industry-wide floppy disk standard

Macintosh External Disk Drive - Apple FDHD Drive
The Apple IIe could not utilize the drive in any form, unless it had the specialized interface card installed, much like the UniDisk 3.5 which the SuperDrive replaced

Macintosh External Disk Drive - Macintosh HDI-20 External 1
Macintosh External Disk Drive - Macintosh HDI-20 External 1.4MB Floppy Disk Drive Manufactured exclusively for use with the Macintosh PowerBook line, the Macintosh HDI-20 External 1.44MB Floppy Disk Drive (M8061) contained a low-powered, slimmer version of the SuperDrive and used a small square HDI-20 proprietary connector, rather than the larger standard DE-19 desktop connector, and was powered directly by the laptop

Hard coding - Startup disk
Some "copy-protected" programs look for a particular file on a floppy disk or flash drive on startup to verify that they are not unauthorized copies. If the computer is replaced by a newer machine, which doesn't have a floppy drive, the program that requires it now can't be run, since the floppy disk can't be inserted.

Hard coding - Startup disk
This last example shows why hard-coding may turn out to be impractical even when it seems at the time that it would work completely. In the 1980s and 1990s the great majority of PCs were fitted with at least one floppy drive, but floppy drives later fell out of use. A program hard-coded in that manner 15 years ago could face problems if not updated.

Disk mirroring - Overview
Typically, mirroring is provided in either hardware solutions such as disk arrays, or in software within the Operating System (such as Linux mdadm and device mapper). Additionally, file systems like Btrfs or ZFS provide integrated data mirroring. There are additional benefits from Btrfs, which maintains both data and metadata integrity checksums, making itself capable of detecting bad copies of blocks, and using mirrored data to pull up data from correct blocks.

Disk mirroring - Overview
The copying of data from one side of a mirror pair to another is called rebuilding or, less commonly, resilvering.

Windows 2000 - Basic and dynamic disk storage
Windows 2000 introduced the Logical Disk Manager and the diskpart command line tool for Logical Disk Manager|dynamic storage. All versions of Windows 2000 support three types of Logical Disk Manager|dynamic disk volumes (along with basic disks): simple volumes, spanned volumes and striped volumes:

* 'Simple volume', a volume with disk space from one disk.

* 'Spanned volumes', where up to 32 disks show up as one, increasing it in size but not enhancing performance. When one disk fails, the array is destroyed. Some data may be recoverable. This corresponds to JBOD#Concatenation (JBOD or SPAN)|JBOD and not to Redundant array of independent disks#RAID 1|RAID-1.

* 'Striped volumes', also known as Redundant array of independent disks#RAID 0|RAID-0, store all their data across several disks in stripes. This allows better performance because disk reads and writes are balanced across multiple disks. Like spanned volumes, when one disk in the array fails, the entire array is destroyed (some data may be recoverable).

In addition to these disk volumes, Windows 2000 Server, Windows 2000 Advanced Server, and Windows 2000 Datacenter Server support mirrored volumes and striped volumes with parity:

* 'Mirrored volumes', also known as Redundant array of independent disks#RAID 1|RAID-1, store identical copies of their data on 2 or more identical disks (mirrored). This allows for fault tolerance; in the event one disk fails, the other disk(s) can keep the server operational until the server can be shut down for replacement of the failed disk.

* 'Striped volumes with parity', also known as Redundant array of independent disks#RAID 5|RAID-5, functions similar to striped volumes/RAID-0, except parity data is written out across each of the disks in addition to the data. This allows the data to be rebuilt in the event a disk in the array needs replacement.

Sony Mavica - Digital still cameras with storage on 3.5 floppy disk
* Sony Digital Mavica MVC-FD73 / MVC-FD73CUSA / MVC-FD73K / MVC-FD73WR (1999, 640 x 480 pixels. fixed ISO 100. F/ mm zoom. Shutter 1/4000 sec to 1/60 sec

* Sony FD Mavica MVC-FD75 / MVC-FD75CUSA / MVC-FD75WAL (2001) (10× optical zoom lens)

* Sony FD Mavica MVC-FD200 (2002) (same as above but 2MP)

Nintendo 64 - Disk Drive This large add on allowed players to play Nintendo 64 disk games that were released by connecting to the N64 through a slot on the bottom of the console

Operating systems - Disk access and file systems
The specific way in which files are stored on a disk is called a file system, and enables files to have names and attributes

A connected data storage device|storage device, such as a Hard Drive, is accessed through a device driver. The device driver understands the specific language of the drive and is able to translate that language into a standard language used by the Operating System to access all disk drives. On UNIX, this is the language of block devices.

Under Windows, each file system is usually limited in application to certain media; for example, CDs must use ISO 9660 or Universal Disk Format|UDF, and as of Windows Vista, NTFS is the only file system which the Operating System can be installed on

Personal computers - Hard disk
The disk drives use a sealed head/disk assembly (HDA) which was first introduced by IBM's Winchester disk system. The use of a sealed assembly allowed the use of positive air pressure to drive out particles from the surface of the disk, which improves reliability.

Personal computers - Hard disk
If the mass storage controller provides for expandability, a PC may also be upgraded by the addition of extra hard disk or optical disc drives. For example, Blu-ray|BD-ROMs, DVD-RWs, and various optical disc recorders may all be added by the user to certain PCs. Standard internal storage device connection interfaces are Parallel ATA|PATA, Serial ATA, and SCSI.

Xbox Exhibition disks 'Xbox Exhibition' was a game demo compilation series from Microsoft Game Studios to advertise and preview upcoming Xbox (console)|Xbox games. The Optical disc|disks contained several playable game demos, game trailers, video content from G4 (TV channel)|G4 TV, music videos, and music from Indie music|indie artists which were downloadable to the Xbox's hard disk.

Xbox Exhibition disks With the advent of the Xbox 360, distribution of demos switched to free downloads from the Xbox Live Marketplace. In total there were seven Xbox Exhibition disks released over two years. The last release was volume 7 on November 17, 2004.

Xbox Exhibition disks - Disks
: Nine playable game demos: Halo (series)|Halo, Madden NFL 2002, NFL Fever 2003, Panzer Dragoon Orta, Quantum Redshift, Splinter Cell, TimeSplitters 2, ToeJam Earl III: Mission to Earth, and Whacked!. Downloads include Dead or Alive 3 booster pack costumes, two downloadable saves for Project Gotham Racing, and two save files for Rallisport Challenge. Music and music videos from: Death Cab for Cutie, John Vanderslice, Rilo Kiley, The Dismemberment Plan, and The Long Winters.

: All-Star Baseball 2004, Capcom vs. SNK 2#Capcom vs. SNK 2 EO|Capcom vs. SNK 2 EO, Indiana Jones and the Emperor's Tomb, Kung Fu Chaos, MechAssault, MX Superfly, NBA Inside Drive 2003, Tom Clancy's Ghost Recon, and Vexx. Downloads were an NFL Fever 2003 roster update, and new characters and environments for ToeJam Earl III: Mission to Earth and MX Superfly.

: Apex (Video Game)|Apex, ATV Quad Power Racing 2, Brute Force (Video Game)|Brute Force, Gladius (Video Game)|Gladius, Inside Pitch 2003, MotoGP 2, NBA Street Vol. 2, Return to Castle Wolfenstein: Tides of War, Tao Feng: Fist of the Lotus. Downloadable levels and skins for RLH: Run Like Hell, and an extra mission for Splinter Cell called the Kola Cell level.

: NBA 2K, Grabbed by the Ghoulies, Magic the Gathering: Battlegrounds, Metal Arms: Glitch in the System, Prince of Persia: Sands of Time, Project Gotham Racing 2, Tom Clancy's Rainbow Six 3, Teenage Mutant Ninja Turtles, and Voodoo Vince. Downloadable content on-disk includes new levels for Ghost Recon: Island Thunder, Return to Castle Wolfenstein: Tides of War, and Star Wars: The Clone Wars, as well as four new maps for Unreal Championship.

: NBA Ballers, Psi-Ops: The Mindgate Conspiracy, Metal Slug 3, Ninja Gaiden (2004 Video Game)|Ninja Gaiden, Shadow Ops: Red Mercury, Rallisport Challenge 2, The Chronicles of Riddick: Escape from Butcher Bay.

: Full Spectrum Warrior, Tom Clancy's Rainbow Six 3: Black Arrow, NCAA Football 2005, Men of Valor, OutRun 2, Second Sight (Video Game)|Second Sight, Blinx 2: Masters of Time and Space

: Seven demos: Burnout 3: Takedown, Dead or Alive Ultimate, ESPN NHL 2K5, Kingdom Under Fire: The Crusaders, Star Wars Republic Commando, The Incredibles (Video Game)|The Incredibles, and The SpongeBob SquarePants Movie Game.

PCGamer - Demo disk Similarly to the British edition, the magazine ships with a Game demo|demo disk, though diskless versions are available. The CDs are replaced by DVDs in the American edition on a month-to-month basis.

PCGamer - Demo disk When PC games with full motion video (FMV) sequences were popular in the mid-to-late 1990s, PC Gamer's CD-ROM included elaborate FMV sequences featuring one of their editors

PCGamer - Demo disk In the September 2011 edition of PC Gamer, it was announced that they would be dropping the demo disk altogether and concentrating on improving the quality of the magazine instead with a promise of a larger magazine printed on heavier paper stock. The usual demo disk content would be made available online. PC Gamer (US), September 2011

Video Disk Recorder 'VDR' ('The Video Disk Recorder') is an open source application for Linux designed to allow any computer to function as a digital video recorder, in order to record and replay TV programming using the computer's hard drive. The computer needs to be equipped with a digital TV tuner card. VDR can also operate as an digital audio player|mp3 player and DVD player using available Plug-in (computing)|plugins. VDR uses drivers from the LinuxTV project.

Synology Inc. - Synology DiskStation Manager (DSM)
As of August 2013, Synology has provided these updates free-of-charge, and they are available for DiskStations for as long as the hardware can support the newer functions of the DSM.

Synology Inc. - Synology DiskStation
The Synology DiskStation is the NAS Computer appliance|appliance which is produced to run the Synology DSM OS. The DiskStation is a pedestal/tower embedded NAS appliance, which ranges from one-bay to as many as twelve bays within a single enclosure. Some models feature storage scalability, where physical units may be tethered together for additional storage, without the need for volume recreation or loss of availability.

Portable media player - SanDisk Sansa
In May 2005, flash memory maker SanDisk entered the PMP market with the SanDisk Sansa|Sansa line of players, starting with the e100 series, and then following up with the m200 series, and c100 series. The Sansa View was slated for 2007 but was shelved before the plans went into effect. Other discontinued series include Connect, Express, Shaker, Clip, Fuze, e200, and c200. The current series include the slotPlayer, Clip Zip, Clip+, and Fuze+.

Aspire One - Hard disks The hard disk is a regular 2.5-in 5400 rpm SATA drive with 80, 120, 160, 250, 320, 500 or 750GB. A number of different drives from different manufacturers have been reported to be included. Newer-model Aspire Ones take a 7mm thick drive, as opposed to the usual 9.5mm thickness that makes up most 2.5-inch form factor hard drives and SSDs.

Western Digital - Parallel ATA hard disk drives
Western Digital is the only remaining manufacturer of parallel ATA hard disk drives for desktop PCs.[ Western Digital Parallel ATA hard disk drives for desktops.]

Data transfer rate (disk drive)
These devices include those with rotating Media (computer)|media, hereby called rotating drives, i.e., hard disk drives (HDD), floppy disk drives (FDD), optical discs (DVD-RW / CD-RW), and it also covers devices without moving parts like solid-state drives (SSD)

Data transfer rate (disk drive) - Access time
The factors that control this time on a rotating drive are mostly related to the mechanical nature of the rotating disks and moving Disk read-and-write head|heads

Data transfer rate (disk drive) - Seek time
The data on the media is stored in sectors which are arranged in parallel circular tracks (concentric or spiral depending upon the Disk storage#CAV-CLV|device type) and there is an actuator with an arm that suspends a head that can transfer data with that media

A rotating drive's average seek time is the average of all possible seek times which technically is the time to do all possible seeks divided by the number of all possible seeks, but in practice it is determined by statistical methods or simply approximated as the time of a seek over one-third of the number of tracks

The first HDD had an average seek time of about 600 ms, and by the middle 1970s, HDDs were available with seek times of about 25ms. Some early PC drives used a stepper motor to move the heads, and as a result had seek times as slow as 80–120ms, but this was quickly improved by voice coil type actuation in the 1980s, reducing seek times to around 20ms. Seek time has continued to improve slowly over time.

The other two less commonly referenced seek measurements are track-to-track and full stroke. The track-to-track measurement is the time required to move from one track to an adjacent track. This is the shortest (fastest) possible seek time. In HDDs this is typically between 0.2 and 0.8ms. The full stroke measurement is the time required to move from the outermost track to the innermost track. This is the longest (slowest) possible seek time.

With SSDs there are no moving parts, so a measurement of the seek time is only testing electronic circuits preparing a particular location on the memory in the storage device. Typical SSDs will have a seek time between 0.08 and 0.16ms.

Data transfer rate (disk drive) - Short stroking
Short stroking is a term used in enterprise storage environments to describe an HDD that is purposely restricted in total capacity so that the actuator only has to move the heads across a smaller number of total tracks

Data transfer rate (disk drive) - Effect of audible noise and vibration control
Measured in A-weighting|dBA, audible noise is significant for certain applications, such as digital video recorder|DVRs, digital audio recording and quiet PC|quiet computers. Low noise disks typically use fluid bearings, slower rotational speeds (usually 5,400rpm) and reduce the seek speed under load (Automatic Acoustic Management|AAM) to reduce audible clicks and crunching sounds. Drives in smaller form factors (e.g. 2.5inch) are often quieter than larger drives.

Data transfer rate (disk drive) - Effect of audible noise and vibration control
Some desktop- and laptop-class disk drives allow the user to make a trade-off between seek performance and drive noise

Data transfer rate (disk drive) - Rotational latency
Therefore the rotational latency and resulting access time can be improved (decreased) by increasing the rotational speed of the disks

The spindle motor speed can use one of two types of disk rotation methods: 1) constant linear velocity (CLV), used mainly in optical storage, varies the rotational speed of the optical disc depending upon the position of the head, and 2) constant angular velocity (CAV), used in HDDs, standard FDDs, a few optical disc systems, and Gramophone record|vinyl audio records, spins the media at one constant speed regardless of where the head is positioned.

Another wrinkle occurs depending on whether surface bit densities are constant

Data transfer rate (disk drive) - Effect of reduced power consumption
Most hard disk drives today support some form of power management which uses a number of specific power modes that save energy by reducing performance

Data transfer rate (disk drive) - Other
The or command overhead is the time it takes for the drive electronics to set up the necessary communication between the various components in the device so it can read or write the data. This is in the range of 0.003Millisecond|ms. With a value this low most people or benchmarks tend to ignore this time.

Data transfer rate (disk drive) - Other
The measures the time it takes the heads to settle on the target track and stop vibrating so it does not read or write off track. This amount is usually very small (typically less than 0.1 ms) or already included in the seek time specifications from the drive manufacturer. In a benchmark test the settle time would be included in the seek time.

Data transfer rate (disk drive) - Data transfer rate
The data transfer rate of a drive (also called throughput) covers both the internal rate (moving data between the disk surface and the controller on the drive) and the external rate (moving data between the controller on the drive and the host system)

; Media rate: Rate at which the drive can read bits from the surface of the media.

; Sector overhead time: Additional time (bytes between sectors) needed for control structures and other information necessary to manage the drive, locate and validate data and perform other support functions.

; Head switch time: Additional time required to electrically switch from one head to another and begin reading; only applies to multi-head drive and is about 1 to 2 ms.

; Cylinder switch time: Additional time required to move to the first track of the next cylinder and begin reading; the name cylinder is used because typically all the tracks of a drive with more than one head or data surface are read before moving the actuator. This time is typically about twice the track-to-track seek time. As of 2001, it was about 2 to 3 ms.

*, a typical 7200RPM desktop HDD has a disk-to-disk buffer|buffer data transfer rate up to 1030Mbit/s. This rate depends on the track location, so it will be higher on the outer zones (where there are more data sectors per track) and lower on the inner zones (where there are fewer data sectors per track); and is generally somewhat higher for 10,000RPM drives.

*Floppy disk drives have sustained disk-to-disk buffer|buffer data transfer rates that are one or two orders of magnitude slower than that of HDDs.

*The sustained disk-to-disk buffer|buffer data transfer rates varies amongst families of Optical disk drives with the slowest CD-ROM#Transfer rates|1x CDs at 1.23Mbit/s floppy-like while a high performance Blu-ray disc#Recording speed|12x Blu-ray disc drive at 432Mbit/s approaches the performance of HDDs.

A current widely used standard for the buffer-to-computer interface is 3.0Gbit/s SATA, which can send about 300megabyte/s (10-bit encoding) from the buffer to the computer, and thus is still comfortably ahead of today's disk-to-buffer transfer rates.

Data transfer rate (disk drive) - Effect of file system
Transfer rate can be influenced by file system fragmentation and the layout of the files. Defragmentation is a procedure used to minimize delay in retrieving data by moving related items to physically proximate areas on the disk. Some computer operating systems perform defragmentation automatically. Although automatic defragmentation is intended to reduce access delays, the procedure can slow response when performed while the computer is in use.

Data transfer rate (disk drive) - Effect of file system
Flash memory-based SSDs do not need defragmentation; however because SSDs write pages of data that are much larger than the blocks of data managed by the file system, over time, an SSD's write performance can degrade as the drive becomes full of partial pages and/or pages no longer needed by the file system

Data transfer rate (disk drive) - Effect of areal density
Simply increasing the number of tracks on a disk can affect seek times but not gross transfer rates

Data transfer rate (disk drive) - Interleave
Without interleaving, the next logical sector would arrive at the read/write head before the equipment was ready, requiring the system to wait for another complete disk revolution before reading could be performed.

However, because interleaving introduces intentional physical delays between blocks of data thereby lowering the data rate, setting the interleave to a ratio higher than required causes unnecessary delays for equipment that has the performance needed to read sectors more quickly. The interleaving ratio was therefore usually chosen by the end-user to suit their particular computer system's performance capabilities when the drive was first installed in their system.

Modern technology is capable of reading data as fast as it can be obtained from the spinning platters, so hard drives usually have a fixed sector interleave ratio of 1:1, which is effectively no interleaving being used.

Data transfer rate (disk drive) - Power consumption
Power consumption can also be reduced by parking the drive heads when the disk is not in use reducing friction, adjusting spin speeds, and disabling internal components when not in use.

Drives use more power, briefly, when starting up (spin-up). Although this has little direct effect on total energy consumption, the maximum power demanded from the power supply, and hence its required rating, can be reduced in systems with several drives by controlling when they spin up.

* On SCSI hard disk drives, the SCSI controller can directly control spin up and spin down of the drives.

* Some Parallel ATA (PATA) and Serial ATA (SATA) hard disk drives support power-up in standby or PUIS: each drive does not spin up until the controller or system BIOS issues a specific command to do so. This allows the system to be set up to stagger disk start-up and limit maximum power demand at switch-on.

* Some SATA II and later hard disk drives support Staggered spinup|staggered spin-up, allowing the computer to spin up the drives in sequence to reduce load on the power supply when booting.

Most hard disk drives today support some form of power management which uses a number of specific power modes that save energy by reducing performance. When implemented an HDD will change between a full power mode to one or more power saving modes as a function of drive usage. Recovery from the deepest mode, typically called Sleep, may take as long as several seconds.

Data transfer rate (disk drive) - Shock resistance
Shock resistance is especially important for mobile devices. Some laptops now include active hard drive protection that parks the disk heads if the machine is dropped, hopefully before impact, to offer the greatest possible chance of survival in such an event. Maximum shock tolerance to date is 350 Gravitational acceleration|g for operating and 1,000 g for non-operating.

AVCHD - Hard disk drive A hard disk drive was added as an optional recording medium to AVCHD specification shortly after the new video standard had been announced. Presently, capacity of built-in HDDs ranges from 30GB to 240GB.

AVCHD - Hard disk drive * Higher capacity than other media types, which allows for longer continuous recording.

AVCHD - Hard disk drive * Sensitive to atmospheric pressure. The HDD may fail if the camcorder is used at altitudes above .

* Vulnerable to mechanical shock or fast movement.
AVCHD - Hard disk drive * Vulnerable to mechanical shock or fast movement.

AVCHD - Hard disk drive * All HDD-based AVCHD camcorders employ non-removable disks. To transfer video to a computer the camcorder must be connected with a USB cable. Most camcorders require using an AC power adapter for this operation.

AVCHD - Hard disk drive * The sound of moving magnetic heads may be heard in the recorded video when recording in quiet environment.

AVCHD - Hard disk drive * Replacing a damaged HDD requires disassembling a camcorder and cannot be done by a consumer.

Digital Multilayer Disk
'Digital Multilayer Disk' ('DMD') is an optical disc format developed by D Data Inc

Digital Multilayer Disk - Advantages
* A high definition movie requires about 13 GB of storage with compression, so it can fit in an HD-DMD single disc, and there is enough space to add additional content and features.

* Because DMD only requires a stable, reliable red laser to read information from the disk, the manufacturing processes for both disks and drives is much simpler and much less expensive than other alternatives, like Blu-ray or HD DVD.

* DMD is highly scalable meaning 20GB, 50GB, 100GB+ disks are possible with little additional research development required.

Universal Disk Format 'Universal Disk Format' ('UDF') is a profile (engineering)|profile of the specification known as 'ISO/IEC 13346' and 'ECMA-167'[ ECMA Volume and File Structure for Write-Once and Rewritable Media using Non-Sequential Recording for Information Interchange] and is an open format|open vendor-neutral file system for computer data storage for a broad range of media

Universal Disk Format But when packet writing to rewriteable media, such as CD-RW, UDF allows files to be created, deleted and changed on-disc just as a general-purpose filesystem would on removable media like floppy disks and USB flash drive|flash drives

Universal Disk Format Multi-session mastering is also possible in UDF, though some implementations may be unable to read disks with multiple sessions.Multi-session mastering has always been part of the UDF specification. See [UDF 2.01/6.10.1], though earlier documents were not very clear that the anchor offsets are specified to be from the last session.

Universal Disk Format - History
The Optical Storage Technology Association standardized the UDF file system to form a common file system for all optical media: both for file system permissions|read-only media and for re-writable optical media. When first standardized, the UDF file system aimed to replace ISO 9660, allowing support for both read-only and writable media. After the release of the first version of UDF, the DVD Consortium adopted it as the official file system for DVD-Video and DVD-Audio.

Universal Disk Format - History
[ OSTA - UDF Specifications]

Universal Disk Format - Revisions
Multiple revisions of UDF have been released:[ Wenguang's Introduction to Universal Disk Format (UDF)]

* [ Revision 1.02] (August 30, 1996). This format is used by DVD-Video discs.

* [ Revision 1.50] (February 4, 1997). Added support for (virtual) rewritability on CD-R/DVD-R media by introducing the VAT structure. Added sparing tables for defect management on rewritable media such as CD-RW, and DVD-RW and DVD+RW.

* [ Revision 2.00] (April 3, 1998). Added support for Stream Files and real-time files (for DVD recording) and simplified directory management. VAT support was extended.

* [ Revision 2.01] (March 15, 2000) is mainly a bugfix release to UDF Many of the UDF standard's ambiguities were resolved in version 2.01.

* [ Revision 2.50] (April 30, 2003). Added the Metadata Partition facilitating metadata clustering, easier crash recovery and optional duplication of file system information: All metadata like nodes and directory contents are written on a separate partition which can optionally be mirrored. This format is used by some versions of Blu-ray Disc|Blu-ray discs.

* [ Revision 2.60] (March 1, 2005). Added Pseudo OverWrite method for drives supporting pseudo overwrite capability on sequentially recordable media. (Some versions of Blu-ray Disc|Blu-ray discs use this format.)

Universal Disk Format - Specifications
The UDF standard defines three file system variations, called builds. These are:

* Plain (Random Read/Write Access). This is the original format supported in all UDF revisions

* Virtual Allocation Table a.k.a. VAT (Incremental Writing). Used specifically for writing to CD-R and (write-once) media

* Spared (Limited Random Write Access). Used specifically for writing to CD-RW and DVD-RW (rewritable) media

Universal Disk Format - Plain build
Introduced in the first version of the standard, this format can be used on any type of disk that allows random read/write access, such as hard disks, DVD+RW and DVD-RAM media. Similarly to other common file system formats, such as File Allocation Table|FAT, directory entries point directly to the block or sector numbers of their file contents. In writing to such a disk in this format, any physical block on the disk may be chosen for allocation of new or updated files.

Universal Disk Format - Plain build
Since this is the basic format, practically any operating system or file system driver claiming support for UDF should be able to read this format.

Universal Disk Format - VAT build
Write-once media such as DVD-R and CD-R have limitations when being written to, in that each physical block can only be written to once, and the writing must happen incrementally. Thus the plain build of UDF can only be written to CD-Rs by pre-mastering the data and then writing all data in one piece to the media, similar to the way an ISO 9660 file system gets written to CD media.

For write-once media, the entire disk is virtualized, making the write-once nature transparent for the user; the disk can be treated the same way one would treat a rewritable disc.

Eventually, after using this scheme for some time, the disk will be full, as free space cannot be recovered by deleting files

Not all drives fully implement version 1.5 or higher of the UDF, and may therefore not be able to handle VAT builds.

Universal Disk Format - Spared (RW) build
Rewriteable media such as DVD-RW and CD-RW have fewer limitations than DVD-R and CD-R media

The plain and VAT builds of the UDF format can be used on rewriteable media, with some limitations

UDF defect management does not apply to systems that already implement another form of defect management, such as Mount Rainier (packet writing)|Mount Rainier for optical discs, or a disk controller for a hard drive.

The tools and drives that do not fully support revision 1.5 of UDF will ignore the sparing table, which would lead them to read the oudated worn-out sectors, leading to retrieval of corrupted data.

Universal Disk Format - Character set
The specification allows for nine character encodings: one by agreement, one specified by Ecma International|ECMA-6 (also known as ASCII), three subsets of ASCII, a subset of ECMA-94 (Latin-1), and various other graphical characters.

Universal Disk Format - Compatibility
Many DVD players do not support any UDF revision other than version Discs created with a newer revision may still work in these players if the ISO 9660 bridge format is used. Even if an Operating System claims to be able to read UDF 1.50, it still may only support the plain build and not necessarily either the VAT or Spared UDF builds.

Mac OS X claims to support Revision 1.50 (see man mount_udf), yet it can only mount disks of the plain build properly and provides no virtualization support at all. It cannot mount UDF disks with VAT, as seen with the Sony Mavica issue. Releases before mount disks with Sparing Table but does not read its files correctly. Version fixes this problem.

Similarly, Microsoft Windows XP Service Pack 2 (SP2) cannot read DVD-RW disks that use the UDF 2.00 sparing tables as a defect management system. This problem occurs if the UDF defect management system creates a sparing table that spans more than one sector on the DVD-RW disk. Windows XP SP2 can recognize that a DVD is using UDF, but Windows Explorer displays the contents of a DVD as an empty folder. A hotfix is available for this and is included in Service Pack 3.

Optical disc authoring - Universal Disk Format
Universal Disk Format (UDF) can be extended with Mount Rainier (packet writing)|Mount Rainier packet writing, making it possible to use the disc like a floppy disk. This allows one to easily delete, create, and modify files, without having to write the whole disc again.

Giant magnetoresistance - Hard disk drives
In hard disk drives (HDDs), information is encoded using magnetic domains, and the direction of their magnetization is associated with the logical levels 0 and 1. There are two recording methods: longitudinal and perpendicular.

Giant magnetoresistance - Hard disk drives
In the longitudinal method, the magnetization is normal to the surface

Stacked Volumetric Optical Disk
The 'Stacked Volumetric Optical Disc' (or SVOD) is an optical disc format developed by Hitachi, Ltd.|Hitachi/Maxell, which uses an array of wafer-thin optical discs to allow data storage.

Stacked Volumetric Optical Disk
Each layer (a thin polycarbonate disc) holds around 9.4GB of information, and the wafers are stacked in layers of 100 or so, giving overall data storage increase of 100x or more.

Distributed file system - Shared-disk / storage area network
Shared-disk file-systems also usually employ some sort of a fencing (computing)|fencing mechanism to prevent data corruption in case of node failures.

The underlying storage area network may use any of a number of block-level protocols, including SCSI, iSCSI, HyperSCSI, ATA over Ethernet (AoE), Fibre Channel, network block device, and InfiniBand.

There are different architectural approaches to a shared-disk file-system. Some distribute file information across all the servers in a cluster (fully distributed). Others utilize a centralized metadata server. Both achieve the same result of enabling all servers to access all the data on a shared storage device.

RAM - RAM disk Software can partition a portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a RAM disk. A RAM disk loses the stored data when the computer is shut down, unless memory is arranged to have a standby battery source.

Database replication - Disk storage replication
Active (real-time) storage replication is usually implemented by distributing updates of a block device to several physical hard disks. This way, any file system supported by the Operating System can be replicated without modification, as the file system code works on a level above the block device driver layer. It is implemented either in hardware (in a disk array controller) or in software (in a device driver).

A replication is extendable across a computer network, so the disks can be located in physically distant locations, and the master-slave database replication model is usually applied

The main characteristic of such cross-site replication is how write operations are handled:

* Synchronization|Synchronous replication - guarantees zero data loss by the means of atomic operation|atomic write operation, i.e

** An often-overlooked aspect of synchronous replication is the fact that failure of remote replica, or even just the interconnection, stops by definition any and all writes (freezing the local storage system). This is the behaviour that guarantees zero data loss. However, many commercial systems at such potentially dangerous point do not freeze, but just proceed with local writes, losing the desired zero recovery point objective.

** The main difference between synchronous and asynchronous volume replication is that synchronous replication needs to wait for the destination server in any write operation.[ Open-E Knowledgebase. What is the difference between asynchronous and synchronous volume replication?] 12 August 2009.

* Asynchronous I/O|Asynchronous replication - write is considered complete as soon as local storage acknowledges it. Remote storage is updated, but probably with a small lag. Performance is greatly increased, but in case of losing a local storage, the remote storage is not guaranteed to have the current copy of data and most recent data may be lost.

* Semi-synchronous replication - this usually means that a write is considered complete as soon as local storage acknowledges it and a remote server acknowledges that it has received the write either into memory or to a dedicated log file. The actual remote write is not performed immediately but is performed asynchronously, resulting in better performance than synchronous replication but offering no guarantee of durability.

** Point-in-time replication - introduces periodic snapshot (computer storage)|snapshots that are replicated instead of primary storage. If the replicated snapshots are pointer-based, then during replication only the changed data is moved not the entire volume. Using this method, replication can occur over smaller, less expensive bandwidth links such as iSCSI or T1 instead of fiber optic lines.

To address the limits imposed by latency, techniques of WAN optimization can be applied to the link.

NoSQL - KV - solid state or rotating disk
* Keyspace (distributed data store)|Keyspace

NoSQL - KV - solid state or rotating disk
* Virtuoso Universal Server|OpenLink Virtuoso

Hard disk drives An HDD consists of one or more rigid (hard) rapidly rotating disks (hard disk platter|platters) with disk read-and-write head|magnetic heads arranged on a moving actuator arm to read and write data to the surfaces.

Hard disk drives Introduced by IBM in 1956, HDDs became the dominant secondary storage device for history of general purpose CPUs|general purpose computers by the early 1960s

Hard disk drives !-- Please do not correct the following paragraph to use binary prefixes, powers of two, powers of 1024, etc.

Hard disk drives The primary characteristics of an HDD are its capacity and Hard disk drive performance characteristics|performance

Hard disk drives The two most common wikt:form factor|form factors for modern HDDs are 3.5-inch in desktop computers and 2.5-inch in laptops. HDDs are connected to systems by standard computer interface|interface cables such as SATA (Serial ATA), USB or SAS (Serial attached SCSI) cables.

Hard disk drives , the primary competing technology for secondary storage is flash memory in the form of solid-state drives (SSDs)

Hard disk drives - History
HDDs were introduced in 1956 as data storage for an IBM real-time transaction processing system|transaction processing computer and were developed for use with general purpose mainframe computer|mainframe and minicomputers. The first IBM drive, the IBM 350|350 RAMAC, was approximately the size of two refrigerators and stored 5 million 6-bit characters (the equivalent of 3.75 million 8-bit bytes or 3.75 MB or megabytes) on a stack of 50 disks.

In 1961 IBM introduced the IBM 1311|model 1311 disk drive, which was about the size of a washing machine and stored two million characters on a removable disk pack. Users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in most computer installations and reached capacities of 300 megabytes by the early 1980s. Non-removable HDDs were called fixed disk drives.

Some high performance HDDs were manufactured with one head per track, e.g., IBM 2305 so that no time was lost physically moving the heads to a track.Microsoft Windows NT Workstation 4.0 Resource Guide © 1995, Chapter 17 – Disk and File System Basics Known as Fixed-Head or Head-Per-Track disk drives they were very expensive and are no longer in production.Computer Organization and Design, 2nd Ed., P. Pal Chaudhuri, © 2006,p. 635

Instead, the first models of Winchester technology drives featured a removable disk module, which included both the disk pack and the head assembly, leaving the actuator motor in the drive upon removal

Like the first removable pack drive, the first Winchester drives used platters in diameter. A few years later, designers were exploring the possibility that physically smaller platters might offer advantages. Drives with non-removable eight-inch platters appeared, and then drives that used a #Form factors|form factor (a mounting width equivalent to that used by contemporary floppy disk drives). The latter were primarily intended for the then-fledgling personal computer ('PC') market.

External HDDs remained popular for much longer on the Apple Macintosh. Every Mac made between 1986 and 1998 has a SCSI port on the back, making external expansion easy; also, toaster Compact Macs did not have easily accessible HDD bays (or, in the case of the Mac Plus, any hard drive bay at all), so on those models, external SCSI disks were the only reasonable option.

Driven by memory storage density|areal density doubling every two to four years since their invention (an observation known as Kryder's law, similar to Moore's Law), HDDs have continuously improved their characteristics; a few highlights include:

* Physical volume of HDD decreasing from (comparable to a large side-by-side refrigerator), to less than , a 100,000-to-1 decrease.

* Price decreasing from about per megabyte to less than $ per megabyte ($90/1.5 terabyte), a greater than 250-million-to-1 decrease.

* Market application expanding from mainframe computers of the late 1950s to most mass storage applications including computers and consumer applications such as storage of entertainment content.

Hard disk drives - Magnetic recording
An HDD records data by magnetizing a thin film of ferromagnetism|ferromagnetic material on a disk. Sequential changes in the direction of magnetization represent binary data bits. The data is read from the disk by detecting the transitions in magnetization. User data is encoded using an encoding scheme, such as run-length limited encoding, which determines how the data is represented by the magnetic transitions.

A typical HDD design consists of a that holds flat circular disks, also called hard disk platter|platters, which hold the recorded data. The platters are made from a non-magnetic material, usually aluminium alloy, glass, or ceramic, and are coated with a shallow layer of magnetic material typically 10–20 nanometer|nm in depth, with an outer layer of carbon for protection. For reference, a standard piece of copy paper is .

The platters in contemporary HDDs are spun at speeds varying from 4,200 revolutions per minute|rpm in energy-efficient portable devices, to 15,000rpm for high-performance servers. The first HDDs spun at 1,200rpm and, for many years, 3,600rpm was the norm. As of December 2013, the platters in most consumer-grade HDDs spin at either 5,400rpm or 7,200rpm.

Information is written to and read from a platter as it rotates past devices called disk read-and-write head|read-and-write heads that operate very close (often tens of nanometers) over the magnetic surface. The read-and-write head is used to detect and modify the magnetization of the material immediately under it.

In modern drives, the small size of the magnetic regions creates the danger that their magnetic state might be lost because of superparamagnetism|thermal effects

Hard disk drives - Error rates and handling
For example, a typical 1 Terabyte|TB hard disk with 512-byte sectors provides additional capacity of about 93 GiB|GB for the Forward error correction|ECC data.

Typical hard disk drives attempt to remap the data in a physical sector that is failing to a spare physical sector, relying on the ECC to recover stored data while the amount of errors in a bad sector is still low enough. The S.M.A.R.T (Self-Monitoring, Analysis and Reporting Technology) feature counts the total number of errors in the entire HDD fixed by ECC, and the total number of performed sector remappings, as the occurrence of many such errors may predict an HDD failure.

An enterprise disk with a Fibre Channel interface, which uses 520 byte sectors to support the Data Integrity Field standard to combat data corruption, specifies similar error rates in 2005.

The worst type of errors are those that go unnoticed, and are not even detected by the disk firmware or the host operating system. These errors are known as silent data corruption, some of which may be caused by hard disk drive malfunctions.

Hard disk drives - Calculation
The gross capacity of older HDDs can be calculated by multiplying for each Zone bit recording|zone of the drive the number of cylinder (disk drive)|cylinders by the number of heads by the number of disk sector|sectors/zone by the number of bytes/sector (most commonly 512) and then summing the totals for all zones

Hard disk drives - Calculation
The old C/H/S scheme has been replaced by logical block addressing. In some cases, to try to force-fit the C/H/S scheme to large-capacity drives, the number of heads was given as 64, although no modern drive has anywhere near 32 platters: the typical 2TB hard disk as of 2013 has two 1TB platters (and 4TB drives use four platters).

Hard disk drives - File system use
The presentation of an HDD to its host is determined by its disk controller|controller. This may differ substantially from the drive's native interface (computing)|interface particularly in mainframe computer|mainframes or server computer|servers.

Hard disk drives - File system use
Examples of data structures stored on disk to retrieve files include the file allocation table (FAT) in the MS-DOS file system and inodes in many UNIX file systems, as well as other operating system data structures

Hard disk drives - Units
The total capacity of HDDs is given by manufacturers in megabytes (1MB=1,000,000 bytes), gigabytes (1GB=1,000,000,000 bytes) or terabytes (1TB=1,000,000,000,000 bytes)

In the case of mega-, there is a nearly 5% difference between the powers of 1,000 definition and the powers of 1,024 definition

In December 1998, Binary prefix#Adoption by IEC and NIST|standards organizations addressed these dual definitions of the conventional prefixes by standardizing on unique binary prefixes and prefix symbols to denote multiples of 1,024, such as mebibyte (MiB), which exclusively denotes 220 or 1,048,576 bytes. This standard has seen little adoption by the computer industry, and the conventionally prefixed forms of byte continue to denote slightly different values depending on context.

Hard disk drives - Form factors
Mainframe and minicomputer hard disks were of widely varying dimensions, typically in free standing cabinets the size of washing machines or designed to fit a 19 inch rack|19 rack. In 1962, IBM introduced its IBM 1311|model 1311 disk, which used 14inch (nominal size) platters. This became a standard size for mainframe and minicomputer drives for many years. Such large platters were never used with microprocessor-based systems.

With increasing sales of microcomputers having built in floppy disk|floppy-disk drives (FDDs), HDDs that would fit to the FDD mountings became desirable. Thus HDD Form factors, initially followed those of 8-inch, 5.25-inch, and 3.5-inch floppy disk drives. Because there were no smaller floppy disk drives, smaller HDD form factors developed from product offerings or industry standards.

; 8inch: × × ( × × ). In 1979, Shugart Associates' SA1000 was the first form factor compatible HDD, having the same dimensions and a compatible interface to the 8 FDD.

Most desktop models of drives for optical 120mm disks (DVD, CD) use the half height 5¼ dimension, but it fell out of fashion for HDDs

; 3.5inch: 4in × 1in × 5.75in (101.6mm × 25.4mm × 146mm) = cm³. This smaller form factor is similar to that used in an HDD by Rodime in 1983, which was the same size as the half height 3½ FDD, i.e., 1.63inches high. Today, the 1-inch high (slimline or low-profile) version of this form factor is the most popular form used in most desktops.

; 2.5inch: × 0.275– × ( × 7– × ) = –

; 1.8inch: 54mm × 8mm × 71mm = cm³. This form factor, originally introduced by Integral Peripherals in 1993, evolved into the ATA-7 LIF with dimensions as stated. For a time it was increasingly used in digital audio players and subnotebooks, but its popularity decreased to the point where this form factor is increasingly rare and only a small percentage of the overall market.

; 1inch: 42.8mm × 5mm × 36.4mm. This form factor was introduced in 1999 as IBM's Microdrive to fit inside a Compact Flash|CF Type II slot. Samsung calls the same form factor 1.3inch drive in its product literature.

; 0.85inch: 24mm × 5mm × 32mm. Toshiba announced this form factor in January 2004 for use in mobile phones and similar applications, including Secure Digital card|SD/MultiMediaCard|MMC slot compatible HDDs optimized for video storage on 4G handsets. Toshiba manufactured a 4GB (MK4001MTD) and an 8GB (MK8003MTD) version and holds the Guinness World Record for the smallest HDD.

Hard disk drives - Time to access data
Delay may also occur if the drive disks are stopped to save energy.

Hard disk drives - Time to access data
Access time|Time to access data can be improved by increasing rotational speed (thus reducing latency) and/or by reducing the time spent seeking

Hard disk drives - Seek time
Average seek time ranges from under 4millisecond|ms for high-end server drives to 15ms for mobile drives, with the most common mobile drives at about 12ms and the most common desktop type typically being around 9ms

Hard disk drives - Data transfer rate
Data transfer rate (read/write) can be measured by writing a large file to disk using special file generator tools, then reading back the file

Hard disk drives - Data transfer rate
While areal density advances by increasing both the number of tracks across the disk and the number of sectors per track, only the latter increases the data transfer rate for a given rpm

Hard disk drives - Integrity and failure
Due to the extremely close spacing between the heads and the disk surface, HDDs are vulnerable to being damaged by a head crash—a hard disk failure|failure of the disk in which the head scrapes across the platter surface, often grinding away the thin magnetic film and causing data loss. Head crashes can be caused by electronic failure, a sudden power failure, physical shock, contamination of the drive's internal enclosure, wear and tear, corrosion, or poorly manufactured platters and heads.

Breather holes can be seen on all disk drives—they usually have a sticker next to them, warning the user not to cover the holes

For giant magnetoresistive effect|giant magnetoresistive (GMR) heads in particular, a minor head crash from contamination (that does not remove the magnetic surface of the disk) still results in the head temporarily overheating, due to friction with the disk surface, and can render the data unreadable for a short period until the head temperature stabilizes (so called thermal asperity, a problem which can partially be dealt with by proper electronic filtering of the read signal).

A common expectation is that hard disk drives designed for server use will fail less frequently than consumer-grade drives usually used in desktop computers. A study by Carnegie Mellon University and an independent one by Google both found that the grade of a drive does not relate to the drive's failure rate.

A 2011 summary of research into SSD and magnetic disk failure patterns by Tom's Hardware summarized research findings as follows:[ Investigation: Is Your SSD More Reliable Than A Hard Drive?] – Tom's Hardware long term SSD reliability review, 2011, final words

* MTBF does not indicate reliability; the annualized failure rate is higher and usually more relevant.

* Magnetic disks do not have a specific tendency to fail during early use, and temperature only has a minor effect; instead, failure rates steadily increase with age.

* S.M.A.R.T. warns of mechanical issues but not other issues affecting reliability, and is therefore not a reliable indicator of condition.

* Failure rates of drives sold as enterprise and consumer are very much similar, although customized for their different environments.

* In drive arrays, one drive's failure significantly increases the short-term chance of a second drive failing.

Hard disk drives - External hard disk drives
External HDDsThese differ from removable disk media, e.g., disk packs or data modules, in that they include, for example, actuators, drive electronics, motors. typically connect via USB; variants using USB2.0 interface generally have slower data transfer rates when compared to internally mounted hard drives connected through SATA. Plug and play drive functionality offers system compatibility and features large storage options and portable design.

External HDDs are usually available as pre-assembled integrated products, but may be also assembled by combining an external enclosure (with USB or other interface) with a separately purchased HDD

, capacities of external HDDs generally range from 160GB to 2TB; common sizes are 160GB, 250GB, 320GB, 500GB, 640GB, 750GB, 1TB, 2TB, 3TB and 4TB. Features such as biometric security or multiple interfaces (for example, Firewire) are available at a higher cost.

There are pre-assembled external hard disk drives that, when taken out from their enclosures, cannot be used internally in a laptop or desktop computer due to embedded USB interface on their printed circuit boards, and lack of SATA (or Parallel ATA) interfaces.

Hard disk drives - Market segments
: They typically store between 60gigabyte|GB and 4TB and rotate at 5,400 to 10,000revolutions per minute|rpm, and have a media transfer rate of 0.5Gbit/s or higher (1GB = 109 bytes; 1Gbit/s = 109 bit/s). , the highest capacity consumer HDDs store 4terabyte|TB.

: They are smaller than their desktop and enterprise counterparts, tend to be slower and have lower capacity

: Typically used with multiple-user computers running enterprise software

: They include drives embedded into digital video recorders and automotive vehicles. The former are configured to provide a guaranteed streaming capacity, even in the face of read and write errors, while the latter are built to resist larger amounts of shock.

Digital video recorders - Hard-disk based digital video recorders
Consumer digital video recorders ReplayTV and TiVo were launched at the 1999 Consumer Electronics Show in Las Vegas, Nevada|Las Vegas, USA

Hard-disk based digital video recorders make the time shifting feature (traditionally done by a Videocassette recorder|VCR) much more convenient, and also allow for trick modes such as pausing live TV, instant replay of interesting scenes, Chase Play|chasing playback where a recording can be viewed before it has been completed, and skipping of advertising. Most DVRs use the Moving Picture Experts Group|MPEG format for compressing the digitized video signals.

Video recording capabilities have become an essential part of the modern set-top box, as TV viewers have wanted to take control of their viewing experiences

In the UK, digital video recorders are often referred to as plus boxes (such as BSKYB's Sky+ and Virgin Media's V+ which integrates an High-definition television|HD capability, and the subscription free Freesat+ and Freeview+)

Many satellite, cable and IPTV companies are incorporating digital video recording functions into their set-top box, such as with DirecTiVo, DISHPlayer/DishDVR, Scientific Atlanta Explorer 8300|Explorer 8xxx from Time Warner, Total Home DVR from ATT U-verse, Motorola 6412 from Comcast and others, Moxi Media Center by Digeo (available through Charter, Adelphia, Sunflower, Bend Broadband, and soon Comcast and other cable companies), or Sky+

In the United States, the FCC has ruled that starting on July 1, 2007, consumers will be able to purchase a set-top box from a third-party company, rather than being forced to purchase or rent the set-top box from their cable company.WIldstrom, W

Optical disk drive In computing, an 'optical disc drive' ('ODD') is a disk drive that uses laser light or electromagnetic waves within or near the light spectrum|visible light spectrum as part of the process of reading or writing data to or from optical discs

Optical disk drive Floppy disk drives, with capacity of 1.44MB, have been made obsolete: optical media are cheap and have vastly higher capacity to handle the large files used since the days of floppy discs, and the vast majority of computers and much consumer entertainment hardware have optical writers

Optical disk drive Disc recording is restricted to storing files playable on consumer appliances (films, music, etc.), relatively small volumes of data (e.g., a standard DVD holds 4.7gigabytes) for local use, and data for distribution, but only on a small-scale; mass-producing large numbers of identical discs is cheaper and faster than individual recording.

Optical disk drive Optical discs are used to Backup|back up relatively small volumes of data, but backing up of entire hard drives, typically containing many hundreds of gigabytes, is less practical than with the smaller capacities available previously. Large backups are often made on external hard drives, as their price has dropped to a level making this viable; in professional environments tape drive|magnetic tape drives are also used.

Optical disk drive - Rotational mechanism
The rotational mechanism in an optical drive differs considerably from that of a hard disk drives, in that the latter keeps a constant angular velocity (CAV), in other words a constant number of revolutions per minute (RPM). With CAV, a higher throughput is generally achievable at the outer disc compared to the inner.

On the other hand, optical drives were developed with an assumption of achieving a constant throughput, in CD drives initially equal to 150 KiB/s

Later CD drives kept the CLV paradigm, but evolved to achieve higher rotational speeds, popularly described in multiples of a base speed. As a result, a 4× drive, for instance, would rotate at RPM, while transferring data steadily at 600 KiB/s, which is equal to 4 × 150 KiB/s.

For DVDs, base or 1× speed is MB/s, equal to 1.32 MiB/s, approximately 9 times faster than the CD base speed. For Blu-ray drives, base speed is 6.74 MB/s, equal to 6.43 MiB/s.

Because keeping a constant transfer rate for the whole disc is not so important in most contemporary CD uses, a pure CLV approach had to be abandoned to keep the rotational speed of the disc safely low while maximizing data rate

Optical disk drive - Limit
Both DVDs and audio CDs are known to explode.

Optical disk drive - Computer interfaces
Most internal drives for personal computers, Server (computing)|servers and workstations are designed to fit in a standard 5.25 drive bay and connect to their host via an Advanced Technology Attachment|ATA or SATA interface

External drives usually have USB or FireWire interfaces. Some portable versions for laptops power themselves from batteries or directly from their interface bus.

Drives with SCSI interface were made, but they are less common and tend to be more expensive, because of the cost of their interface chipsets, more complex SCSI connectors, and small volume of sales.

Some computers such as the IBM PS/2 were standardizing on the 3.5 floppy and 3.5 hard disk, and did not include a place for a large internal device

This was solved through several techniques:

* Early sound cards could include a CD-ROM drive interface. Initially, such interfaces were proprietary to each CD-ROM manufacturer. A sound card could often have two or three different interfaces.

* A parallel port external drive was developed that connected between a printer and the computer. This was slow but an option for laptops

* A SCSI card could be installed in desktop PCs for an external SCSI drive enclosure, though SCSI was typically much more expensive than other options

Optical disk drive - Internal mechanism of a drive
The DVD-ROM drive in the photo is shown right side up; the disc would sit on top of it. The laser and optical system scans the underside of the disc.

With reference to the photo, just to the right of image center is the disc spin motor, a gray cylinder, with its gray centering hub and black resilient drive ring on top

The gray metal chassis is shock-mounted at its four corners to reduce sensitivity to external shocks, and to reduce drive noise from residual imbalance when running fast. The soft shock mount grommets are just below the brass-colored washers at the four corners (the left one is obscured). Running through those grommets are screws to fasten them to the black plastic frame that's underneath.

Two parallel precision guide rods that run between upper left and lower right in the photo carry the sled, the moving optical read-write head. As shown, this sled is close to, or at the position where it reads or writes at the edge of the disc.

A dark gray disc with two holes on opposite sides has a blue lens surrounded by silver-colored metal

To select tracks (or files) as well as advancing the sled during continuous read or write operations, a motor (it could be a stepping motor; one would hear it if it were) rotates a coarse-pitch leadscrew to move the sled throughout its total travel range

The irregular orange material is flexible etched copper foil supported by thin sheet plastic; these are flexible printed circuits that connect everything to the electronics (which is not shown).

Optical disk drive - Recorder Unique Identifier
Owing to pressure from the music industry, as represented by the International Federation of the Phonographic Industry|IFPI and Recording Industry Association of America|RIAA, Philips developed the Recorder Identification Code (RID) to allow media to be uniquely associated with the recorder that has written it

Optical disk drive - Recorder Unique Identifier
Although the RID was introduced for music and video industry purposes, the RID is included on every disc written by every drive, including data and backup discs. The value of the RID is questionable as it is (currently) impossible to locate any individual recorder due to there being no database.

Optical disk drive - Source IDentification Code
The Source IDentification Code (SID) is an eight character supplier code that is placed on optical discs by the manufacturer. The SID identifies not only manufacturer, but also the individual factory and machine that produced the disc.

Optical disk drive - Source IDentification Code
According to Phillips, the administrator of the SID codes, the SID code provides an optical disc production facility with the means to identify all discs mastered and/or replicated in its plant, including the specific Laser Beam Recorder (LBR) signal processor or mould that produced a particular stamper or disc.

Optical disk drive - Use of RID and SID together in forensics
The standard use of RID and SID mean that each disc written contains a record of the machine that produced a disc (the SID), and which drive wrote it (the RID). This combined knowledge may be very useful to law enforcement, to investigative agencies, and to private and/or corporate investigators.

Disk encryption 'Disk encryption' is a technology which protects information by converting it into unreadable code that cannot be deciphered easily by unauthorized people. 'Disk encryption' uses disk encryption software or disk encryption hardware|hardware to encryption|encrypt every bit of data that goes on a disk storage|disk or disk volume (computing)|volume. Disk encryption prevents unauthorized access to data storage.

Disk encryption Some hardware-based full disk encryption systems can truly encrypt an entire boot disk, including the MBR.

Disk encryption - Disk encryption vs. filesystem-level encryption
Conventional file and folder encryption instead allows different keys for different portions of the disk

Disk encryption - Disk encryption vs. filesystem-level encryption
Unlike disk encryption, filesystem-level encryption does not typically encrypt filesystem metadata, such as the directory structure, file names, modification timestamps or sizes.

Disk encryption - Disk encryption and Trusted Platform Module
Trusted Platform Module (TPM) is a secure cryptoprocessor embedded in the motherboard that can be used to authentication|authenticate a hardware device. Since each TPM chip is unique to a particular device, it is capable of performing platform authentication. It can be used to verify that the system seeking the access is the expected system.

A limited number of disk encryption solutions have support for TPM. These implementations can wrap the decryption key using the TPM, thus tying the hard disk drive (HDD) to a particular device. If the HDD is removed from that particular device and placed in another, the decryption process will fail. Recovery is possible with the decryption password or security token|token.

Although this has the advantage that the disk cannot be removed from the device, it might create a single point of failure in the encryption. For example, if something happens to the TPM or the motherboard, a user would not be able to access the data by connecting the Hard Drive to another computer, unless that user has a separate recovery key.

Disk encryption - Implementations
Hardware-based full disk encryption within the storage device are called self-encrypting drives and have no impact on performance whatsoever

Disk encryption - Password/data recovery mechanism
Secure and safe recovery mechanisms are essential to the large-scale deployment of any disk encryption solutions in an enterprise. The solution must provide an easy but secure way to recover passwords (most importantly data) in case the user leaves the company without notice or forgets the password.

Disk encryption - Challenge/response password recovery mechanism
Challenge-response authentication|Challenge/Response password recovery mechanism allows the password to be recovered in a secure manner. It is offered by a limited number of disk encryption solutions.

# No need for the user to carry a disc with recovery encryption key.

# No secret data is exchanged during the recovery process.

# Does not require a network connection, i.e. it works for users that are at a remote location.

Disk encryption - Emergency Recovery Information (ERI) file password recovery mechanism
An Emergency Recovery Information (ERI) file provides an alternative for recovery if a challenge response mechanism is unfeasible due to the cost of helpdesk operatives for small companies or implementation challenges.

# Small companies can use it without implementation difficulties
Disk encryption - Emergency Recovery Information (ERI) file password recovery mechanism # Small companies can use it without implementation difficulties

Disk encryption - Security concerns
Even a Trusted Platform Module (TPM) is not effective against the attack, as the Operating System needs to hold the decryption keys in memory in order to access the disk.

All software-based encryption systems are vulnerable to various side channel attacks such as acoustic cryptanalysis and hardware keyloggers.

In contrast, self-encrypting drives are not vulnerable to these attacks since the hardware encryption key never leaves the disk controller.

Disk encryption - Benefits
Full disk encryption has several benefits compared to regular file or folder encryption, or encrypted vaults. The following are some benefits of disk encryption:

# Nearly everything including the swap space and the temporary files is encrypted. Encrypting these files is important, as they can reveal important confidential data. With a software implementation, the bootstrapping code cannot be encrypted however. (For example, BitLocker Drive Encryption leaves an unencrypted volume (computing)|volume to booting|boot from, while the volume containing the Operating System is fully encrypted.)

# With full disk encryption, the decision of which individual files to encrypt is not left up to users' discretion. This is important for situations in which users might not want or might forget to encrypt sensitive files.

# Immediate data destruction, as simply destroying the cryptography keys renders the contained data useless. However, if security towards future attacks is a concern, data remanence|purging or physical destruction is advised.

Disk encryption - The boot key problem
Most Full Disk Encryption solutions utilize Pre-Boot Authentication by loading a small, highly secure Operating System which is strictly locked down and hashed versus system variables to check for the integrity of the Pre-Boot kernel

With a Pre-Boot Authentication environment, the key used to encrypt the data is not decrypted until an external key is input into the system.

Solutions for storing the external key include:

* Using a biometric authentication method such as a fingerprint

* Using a dongle to store the key, assuming that the user will not allow the dongle to be stolen with the laptop or that the dongle is encrypted as well.

* Using a boot-time driver that can ask for a password from the user

* Using a network interchange to recover the key, for instance as part of a Preboot Execution Environment|PXE boot

* Using a Trusted Platform Module|TPM to store the decryption key, preventing unauthorized access of the decryption key or subversion of the boot loader.

* Use a combination of the above

All these possibilities have varying degrees of security, however most are better than an unencrypted disk.

Comparison of disk encryption software
This is a technical feature 'comparison of different disk encryption software'.

Comparison of disk encryption software - Features
* 'Hidden containers': Whether hidden containers (an encrypted container (A) within another encrypted container (B) so the existence of container A can not be established)[ Hidden containers description from Jetico (BestCrypt) can be created for deniable encryption. Note that some Block cipher modes of operation|modes of operation like CBC with a plain IV can be more prone to watermarking attacks than others.

* 'Pre-boot authentication': Whether authentication can be required before booting the computer, thus allowing one to encrypt the boot disk.

* 'Custom authentication': Whether custom authentication mechanisms can be implemented with third-party applications.

* 'Multiple keys': Whether an encrypted volume can have more than one active Key (cryptography)|key.

* 'Passphrase strengthening': Whether key strengthening is used with plain text passwords to frustrate dictionary attacks, usually using PBKDF2.

* 'Trusted Platform Module': Whether the implementation can use a TPM cryptoprocessor.

* 'Filesystems': what filesystems are supported.

* 'Two-factor authentication': Whether optional security tokens (Hardware Security Module|hardware security modules, such as Aladdin eToken and smart cards) are supported (for example using PKCS11|PKCS#11)

Comparison of disk encryption software - Layering
* 'Whole disk': Whether the whole physical disk or logical volume can be encrypted, including the partition tables and master boot record. Note that this does not imply that the encrypted disk can be used as the boot disk itself; refer to pre-boot authentication in the features comparison table.

* 'Partition': Whether individual disk partitions can be encrypted.

* 'File': Whether the encrypted container can be stored in a file (usually implemented as encrypted loop devices).

* 'Swap space': Whether the swap space (called a pagefile on Windows) can be encrypted individually/explicitly.

* 'Hibernation file': Whether the Hibernate (OS feature)|hibernation file is encrypted (if hibernation is supported).

Comparison of disk encryption software - Modes of operation
Different modes of operation supported by the software. Note that an encrypted volume can only use one mode of operation.

* 'CBC with predictable IVs': The CBC (cipher block chaining) mode where initialization vectors are statically derived from the sector number and are not secret; this means that IVs are re-used when overwriting a sector and the vectors can easily be guessed by an attacker, leading to watermarking attacks.

* 'CBC with secret IVs': The CBC mode where initialization vectors are statically derived from the encryption key and sector number. The IVs are secret, but they are re-used with overwrites. Methods for this include ESSIV and encrypted sector numbers (CGD).

* 'CBC with random per-sector keys': The CBC mode where random keys are generated for each sector when it is written to, thus does not exhibit the typical weaknesses of CBC with re-used initialization vectors. The individual sector keys are stored on disk and encrypted with a master key. (See GBDE for details)

* 'LRW': The Liskov-Rivest-Wagner tweakable narrow-block mode, a mode of operation specifically designed for disk encryption. Superseded by the more secure XTS mode due to security concerns.IEEE P1619#LRW issue|LRW_issue

Hard disk - History Instead, the first models of Winchester technology drives featured a removable disk module, which included both the disk pack and the head assembly, leaving the actuator motor in the drive upon removal

Hard disk - Error rates and handling
Typical hard disk drives attempt to remap the data in a physical sector that is failing to a spare physical sector provided by the drive's spare sector pool (also called reserve pool), while relying on the ECC to recover stored data while the amount of errors in a bad sector is still low enough

The No-ID Format, developed by IBM in the mid-1990s, contains information about which sectors are bad and where remapped sectors have been located.

[ The PC Guide: Hard Disk: Sector Format and Structure].

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