What do you mean by external storage devices? External storage devices are the devices that temporarily store information for transporting from computer to computer. Such devices are not permanently fixed inside a computer or can be defined as “ storage devices that are not a part of computer. ”

Why there is need of external storage devices? 1. Semiconductor memories are not sufficient to provide the whole storage capacity required in computers. The major limitation in using semiconductor memories is the cost per bit of the stored information. So to fulfil the large storage requirements of computers, magnetic disks, optical disks are generally used.

Advantages of External Storage Devices 1. External storage devices provides additional storage other than that available in computer. 2. Data can be transported easily from one place to another. 3. It is useful to store software and data that is not needed frequently. 4. External storage also works as data back up. 5. This back up may prove useful at times such as fire or theft because important data is not lost.

Types of external storage

One of the oldest type of portable storage devices. Made up of a flexible substance called Mylar. Have a magnetic surface which allows the recording of the data. A standard one can store upto 1.44MB of data which is approx. equivalent to 300 A4 pages of text. A floppy is divided into many concentric circles called tracks and each track is further sub-divided into smaller sections called sectors. FLOPPY DISKS

HARD DISKS Information is stored on one or more circular platters which are continually spinning. Information is recorded on the surface of the rotating disks by magnetic heads as tiny magnetic spots. Info is recorded in bands and each band of information on a given disk is called a track. The tracks on the disk surface are divided into invisible segments called as Sectors

Compact Disks A relatively cheap optical media with capacity of upto 700 MB. Types of CDs :-  CD-ROM: It is compact disk read only memory. It can be read only.  CD-Recordable: It was invented in 1990s. Using CD-R it is possible to write data once on a disk. these are write once read many disks.  CD-Rewritable: There is a limit on how many times a CD-RW can be written. Presently this limit is 1000 times. CD-RW drives are compatible with CD-ROM and CD-R.

DVD DVD stands for digital versatile disk. Its speed is much faster than CD but not as fast as hard disk. The standard DVD-5 technology has a storage capacity of 4.7 GB. The storage capacity changes with the standard used. Its storage capacity (4.7 GB) is much higher than a CD (700 MB). It is achieved by a number of design changes. Types of DVDs :-  DVD-R A write-once format that is compatible with many existing DVD Players, Recorders and DVD-ROM drives. Can only be used in DVD Recorders and Burners that support DVD-R recording or multi-format recording (drives that record "plus" or "dash"). Holds 4.7GB of data or video. Typically, it can hold 2 hours of MPEG-2 video on standard (SP) speed setting.  DVD-RW DVD-RW is the re-writable version of DVD-R. It allows for approximately 1,000 re-writes before it is used up. In general, DVD-RW discs are slightly less compatible than DVD-R. Can only be used in DVD Recorders and Burners that support DVD-RW recording or multi-format recording (drives that record "plus" or "dash"). Also holds 4.7GB of data or video.

 DVD+R These discs are basically the same as DVD-R discs. They hold 4.7GB of data or video and are compatible with most DVD Players and DVD-ROM drives. They can only be used in DVD Recorders & Burners that support DVD+R or multi-format recorders.  DVD+RW The re-writable version of DVD+R. It can record approximately 1,000 times. They also hold 4.7GB of data or video & must be used in DVD+RW compatible Recorders and Burners or multi-format recorders.  DVD-RAM DVD-RAM comes in two varieties & storage capacities. These discs come in both cartridge & non-cartridge varieties & come single-sided/double-sided. Offered by only a few manufacturers (Panasonic, Toshiba & a few other minor ones), DVD-RAM is useful if used like a hard drive. Because it supports & incredible 100,000 re-writes, you can use the disc to record TV shows, view them & then re-write over them many times. Single sided discs hold 4.7GB, double-sided 9.4GB, allowing for longer recording times. DVD- RAM is the least compatible of the 5 recording formats, & is typically used for recording and playback in the same Set Top DVD Recorder.

FLASH MEMORY 1. Flash memory is a solid state memory. It was invented in 1980s by Toshiba. 2. A flash memory is a particular type of EEPROM (Electrically Erasable Programmable Read Only Memory). 3. It is a no-volatile memory. It retains the stored information without requiring a power source. It is called as solid state memory because it has got on moving parts Flash memory is different from the regular EEPROM. 4. In case of EEPROM data are erased one byte at a time which makes it extremely slower. On the other hand data stored in flash memory can be erased in blocks. 5. ` That’s why it gets a name “flash memory” because the chip is organised in such a way that a block of memory cells can be erased at a single time or “flash”.

Advantages of flash memory It has got no moving parts. So it is durable and less susceptible to mechanical damages. It is small in size and light in weight. Hence it is extensively used in portable devices. Flash memory transfers data at a faster rate. As erasing of information in blocks is possible, flash memories are useful in devices where frequent updating of data is required Disadvantages of flash memory The cost of flash memory is high as compared to hard disk. Memory card (for example, CompactFlash) with a 192MB capacity typically costs more than a hard drive with a capacity of 40 GB. The storage capacity of a flash memory is far less than a hard disk.

Flash memory devices Memory card: Memory cards are flash memory storage media used to store digital information in many electronics products. The types of memory cards include Compact Flash, PCMCIA, secure digital card, multimedia card, memory stick etc. Memory stick: Sony introduced memory stick standard in Memory stick is an integrated circuit designed to serve as a storage and transfer medium for digital data. It can store data in various form as text, graphics, digital images etc. transfer of data is possible between devices having memory stick slots. Memory sticks are available in various storage sizes ranging from 4MB to 512MB. The dimensions of a memory stick are 50mm long, 21.5mm wide and 2.8mm thick (in case of pro format). The transfer speed of memory stick is 160 Mbit/s.