A device with read/write heads and associated electronics that can store and retrieve data from one or more rapidly rotating magnetic disks. The disks may be hard disks or floppy disks. The term can also be applied to devices operating with optical disks.

In a magnetic disk drive the data is recorded on one or both sides of a disk in a set of concentric tracks, which are usually subdivided into sectors. The read/write heads of the disk drive are mounted on arms that can be moved, by means of an actuator, to position the heads accurately over the required track. As the disk rotates, the sectors on that track are made accessible. Storage locations can be accessed directly and in any order; typical seek times are in the range 5–12 milliseconds while the latency may be 6 milliseconds. The data is encoded according to a particular disk format. See also access time, disk array, fixed disk drive, floppy-disk drive, RAID.