5 Backing Storage Backing storage is used to store programs and data when they are not being used or when a computer is switched off. When programs and data are needed they are copied into main memory but also still remain on backing storage. Magnetic tape drives, floppy disk drives and hard disk drives are all examples of backing storage devices.

Hard disk drives A hard disk is a circular metal disk coated with magnetic material and usually sealed in a hard disk drive inside the computer. Some hard disk drives are not permanently fixed inside the computer but are removable. Data stored on a hard disk can be accessed much more quickly than data stored on a floppy disk. Hard disks can store much more data than a floppy disk. A typical hard disk inside a personal computer can hold several gigabytes of data.

Floppy disks A floppy disk a circular piece of plastic coated with a magnetic material and protected by a hard plastic cover. The size of a floppy disk is measured in inches. Modern floppy disks are 3½ inches wide. A standard floppy disk can store up to 1.44 Mb of data. Before any type of magnetic disk can be used it must be formatted.

The formatting process involves: Dividing the surface of the disk into invisible circles called tracks and sectors. Setting up a root directory where the list of files that are on the disk will be kept. Data on a magnetic disk is located by finding the address of its location from an index in the root directory. Each address in the index contains a track and sector number for an individual data segment.

CD-ROM CD-ROM stands for compact disk read only memory. A CD-ROM looks just like an ordinary compact disk. CD-ROMs can store approximately 650 megabytes of data which is four hundred times more data than an ordinary 3½ inch floppy disk. CD-ROM disks come with information already on them and are read only. This means that the information on a CD-ROM cannot be erased or changed, and no new information can be saved.

Writeable CDs Writeable compact disks are supplied blank and can have data put onto them using a special read/write CD drive. There are two main types of writeable compact disk; WORM (Write-Once, Read-Many) disks which can have data written to them just once. Magneto-Optical disks which can have data written to them any number of times just like a hard disk.

Magnetic tape Magnetic tape comes in two forms; tape reels, and cassettes or cartridges. Large tape reels are used to make backup copies of programs and data on large mainframe computers. Cartridges are used to make backup copies of the programs and data on personal computers and networks. The main advantage of using magnetic tape as backing storage is that it is relatively cheap and can store large amounts of data.

Direct and serial access Floppy disks, hard disks and CDs all allow direct access to data. Direct access means that the required data can be found straight away without having to read through all the data on the disk. Magnetic tape allows only serial access to data. To locate data on a magnetic tape it has to be searched from the beginning until the required data is found.

File compression File compression software can be used to make files smaller so that more data can be stored in the same amount of space on backing store. When a compressed file on backing store needs to be used it must be decompressed. This can be done using decompression software or by setting files up to be self-extracting which means that they can automatically decompress themselves. Winzip is an example of software that can be used to compress and decompress files.