WordNet (r) 3.0 (2006):

removable disk
    n 1: a hard disk that can be removed from the disk drive;
         removal prevents unauthorized use

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

removable hard disk
removable disk

    A type of magnetic disk, or possibly
   magneto-optical disk which is not permanently attached to
   the disk drive (not a fixed disk) but which can be taken
   out and replaced, allowing many disks to be used in the same
   drive.

   The term "removable disk" would seem to be applicable to
   floppy disks but is generally reserved for hard disks in
   suitable cartridges such as those made by Syquest, Iomega
   and others.

   Removable disk packs were common on minicomputers such as
   the PDP-11 in use in the 1970s except that the drives were
   the size of washing machines and the disk packs as big as
   car wheels.  Removable disks became popular on
   microcomputers in the 1990s as a cheap way of expanding disk
   space, transporting large amounts of data between computers
   and storing backups.  Large, cheap fixed hard disks and USB
   memory sticks have made removable disks less attractive.

   (2007-06-14)