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

disk
disks

    1. magnetic disk.

   2. compact disc.

   3. optical disk.

   Note: the american spelling, "disk", is normal for most
   computer disks whereas "compact disc", having come to
   computers via the audio world, is correctly spelled with a
   "c", indeed, this spelling is part of the CD standard.

   (1995-07-30)