Source : WordNet®

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

Source : Free On-Line Dictionary of Computing

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.
     
        {Floppy disks} are removable disks but the term is not
        commonly used for them but mostly for {hard disks} in suitable
        cartridges such as those made by {Syquest}, {Iomega} and
        others.
     
        Removable disks have become popular on {microcomputers} in the
        1990s since they offer a cheap way of expanding disk space,
        transporting large amounts of data between computers and
        storing back-ups.  The concept is not new however, 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.
     
        (1997-06-06)