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The AP Stylebook czars put out a notice today that included an entry on disc vs disk.
disc, disk Use the disc spelling for phonograph records and related terms (disc jockey), optical and laser-based devices (a Blu-ray disc) and for disc brake. Use disk for computer-related references and medical references, such as a slipped disk.
This is an updated entry in the online version of the venerable style guide. There you have it, the definitive answer in disc vs disk..