The Free On-line Dictionary of Computing (30 December 2018):

disk drive
FDD
floppy disk drive
floppy drive

    (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.

   Two common interfaces for disk drives (and other devices) are
   SCSI and IDE.  ST-506 used to be common in
   microcomputers (in the 1980s?).

   (1997-04-15)