disk

A hard disk. Data is stored in sectors within cylinders and is read by a head which passes over the spinning surface of each disk.

In computing, a common medium for storing large volumes of data. A magnetic disk is rotated at high speed in a disk-drive unit as a read/write (playback or record) head passes over its surfaces to record or read the magnetic variations that encode the data. Optical disks, such as CD-ROM (compact-disc read-only memory) and WORM (write once, read many times), are also used to store computer data. Data are recorded on the disk surface as etched microscopic pits and are read by a laser-scanning device.

Magnetic disks come in several forms: fixed hard disks are built into the disk-drive unit, occasionally stacked on top of one another. A fixed disk cannot be removed: once it is full, data must be deleted in order to free space or a complete new disk drive must be added to the computer system in order to increase storage capacity. Arrays of such disks are also used to store minicomputer and mainframe data in RAID storage systems, replacing large fixed disks and removable hard disks.

Removable hard disks are still found in mid-range and large (mainframe) computer systems. The disks are contained, individually or as stacks (disk packs), in a protective plastic case, and can be taken out of the drive unit and kept for later use. By swapping such disks around, a single hard-disk drive can be made to provide a potentially infinite storage capacity. However, access speeds and capacities tend to be lower that those associated with large, fixed hard disks. A floppy disk (or diskette) is a most common form of backing store for microcomputers, although it is increasingly being replaced by the higher capacity and more robust memory stick or pen-drive or other back-up devices. The floppy disk is much smaller in size and capacity than a hard disk, normally holding 0.5–2 megabytes of data. The floppy disk is so called because it is manufactured from thin flexible plastic coated with a magnetic material. The earliest form of floppy disk was packaged in a card case and was easily damaged; subsequent versions are contained in a smaller, rigid plastic case and are much more robust. All floppy disks can be removed from the drive unit.