In engineering (electrical, not mechanical) school, I was taught that disk refers to storage media that is in protective housing and disc refers to storage media that is not. So, 3.5" floppies and hard drives are disks, while CDs, DVDs, and phonographic records are discs. Now, however, I think this "rule" does not quite paint the complete picture...
My understanding is that disc and disk were originally interchangeable; although, disc was preferred over disk by everyone except for Americans (in a way similar to the examples D’Ecosse mentioned, I assume). Then, for some reason, phonographic records were called discs, even by Americans. Then, CDs were invented and also called discs. I’m not sure if this was a result of being in the music industry (like records, which are called discs) or because the co-inventors of the CD were a European and Japanese company (would use disc over disk) or for some other reason. Whatever the reason, when the CD format was expanded to store computer data and when DVDs were introduced, "disc" naturally stuck. Search google for "disk vs. disc" for more info.