<hardware, storage> (Or "hard disk drive", "hard drive", "floppy disk drive", "floppy drive") A peripheral device that reads and writes hard disks or floppy disks. The drive contains a motor to rotate the disk at a constant rate and one or more read/write heads which are positioned over the desired track by a servo mechanism. It also contains the electronics to amplify the signals from the heads to normal digital logic levels and vice versa.
In order for a disk drive to start to read or write a given location a read/write head must be positioned radially over the right track and rotationally over the start of the right sector.
Radial motion is known as "seeking" and it is this which causes most of the intermittent noise heard during disk activity. There is usually one head for each disk surface and all heads move together. The set of locations which are accessible with the heads in a given radial position are known as a "cylinder". The "seek time" is the time taken to seek to a different cylinder.
The disk is constantly rotating (except for some floppy disk drives where the motor is switched off between accesses to reduce wear and power consumption) so positioning the heads over the right sector is simply a matter of waiting until it arrives under the head. With a single set of heads this "rotational latency" will be on average half a revolution but some big drives have multiple sets of heads spaced at equal angles around the disk.
If seeking and rotation are independent, access time is seek time + rotational latency. When accessing multiple tracks sequentially, data is sometimes arranged so that by the time the seek from one track to the next has finished, the disk has rotated just enough to begin accessing the next track.
See also sector interleave.
Early disk drives had a capacity of a few megabytes and were housed inside a separate cabinet the size of a washing machine. Over a few decades they shrunk to fit a terabyte or more in a box the size of a paperback book.
The disks may be removable disks; floppy disks always are, removable hard disks were common on mainframes and minicomputers but less so on microcomputers until the mid 1990s(?) with products like the Zip Drive.
A CD-ROM drive is not usually referred to as a disk drive.