NGS Style Manual‎ > ‎- D -‎ > ‎

disc, disk

Use disc for a phonograph record, a compact disc, compact disc player (no hyphen), other optical or laser discs, disc brakes, and certain farm equipment:
compact disc
phonograph disc
videodisc
laser disc

CD-ROMs are discs
disc jockey
disc brake
disc harrow
In most other terms, including computer-related ones, use disk:
computer disk
disk drive
diskette
floppy disk
hard disk
magnetic disk
intervertebral disk
slipped disk
moon's disk
solar disk
disk flower