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, DVD, and Blu-Ray are also used to store computer data, which 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 than those associated with large, fixed hard disks.
Floppy disks used to be the most common form of backing store for microcomputers, but have now for most users been superseded by the higher capacity and more robust USB memory stick.