The Jargon File (version 4.4.7, 29 Dec 2003):

dongle-disk
 /don'gl disk/, n.

    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.) Also called a key disk. See dongle.


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

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)