The Collaborative International Dictionary of English v.0.48:

magnetic disc \magnetic disc\, magnetic disk \magnetic disk\n.
   A ditical memory device consisting of a flat disk covered
   with a magnetic coating on which information is stored; a
   hard disk, floppy disk, and diskette are typically
   magnetic disks.

   Syn: disk, disc.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

hard disk
    n 1: a rigid magnetic disk mounted permanently in a drive unit
         [syn: hard disc, hard disk, fixed disk]

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

hard disk

    (In contrast to floppy disk) A magnetic disk
   data storage device where the disks are rigid and fixed to a
   central axle.  They are usually packaged with associated
   read/write heads and electronics.  Most hard disks are
   permanently connected to the drive (fixed disks) though there
   are also removable hard disks.

   See magnetic disk.

   (2007-06-14)