Disk is (or can be) short for diskette - which means a disc in a protective sheath. Meanwhile a "disc" is any flat circular object. So floppy disk, hard disk,

Disk is (or can be) short for diskette - which means a disc in a protective sheath. Meanwhile a "disc" is any flat circular object. So floppy disk, hard disk, but compact disc, is correct. The only medium I know of that doesn't obey this rule is the Sony MiniDisc. MDs are definitely diskettes, and so should in theory be spelled MiniDisk. But presumably that's for trademark reasons.