Source : Free On-Line Dictionary of Computing

dongle-disk
     
        /don'gl disk/ (Or "key disk") A kind of {dongle} consisting of
        a special {floppy disk} that is required in order to perform
        some task.  Some contain special coding that allows an
        application to identify it uniquely, others *are* special code
        that does something that normally-resident programs don't or
        can't.
     
        For example, {AT&T}'s "Unix PC" would only come up in {root
        mode} with a special boot disk.
     
        [{Jargon File}]
     
        (1998-12-13)