Storage devices

The electronic memory inside a computer is of limited capacity and can only hold data when the computer is switched on. A storage device is used to store data that is not being processed and to save data when the computer is switched off. There is a variety of storage devices and storage media available. These include magnetic devices (e.g. floppy disk drives, hard disk drives, tape drives), optical devices (e.g. CD-ROM and DVD drives), and magneto-optical drives.

Disks have to be treated with care if you do not want to damage them or the data stored on them. Damage can be caused by physical strain, dust, smoke particles, fingermarks, sunlight, heat, and magnetism, depending on the type of media used.

When in use, a disk rotates at high speed and a read/write head is brought very close to its surface. If the disk is removed when the drive is in use, the read/write head and the disk surface may be damaged. If extra labels are attached to the disk, it can very easily get stuck in the drive. In this case, it should not be removed forcibly in case the drive mechanism or the surface of the disk is damaged. A hard disk is particularly easily damaged because it contains more than one disk and read/write head.

The read/write heads are extremely close to the surfaces of the disks and the disks spin at a very high speed. Even a small smoke or dust particle can destroy the drive. It is therefore enclosed in a vacuum sealed case. If a hard disk suddenly fails completely, the disk is said to have crashed.

When comparing storage devices, the following factors have to be taken into account. What is the storage cost per megabyte? How fast are they at reading and writing data? What is their maximum storage capacity? Are they used by the people you need to exchange data with? Do they conform to a standard? Are they fixed or removable? Are they read only or read and write? Do they use random or serial access?

It is important to keep backup copies of stored data. Magnetic tape is often used because it is very cheap and can hold extremely large amounts of data. It is common to use a backup scheme where a number of tapes are used in rotation.