[Markoff]

> In most varieties of English, disk is the preferred spelling for magnetic media (hence floppy disk, hard disk, disk drive), whereas disc is preferred for optical media (hence compact disc, digital versatile disc, optical disc).

that's exactly my understanding as a non-native English speaker who haven't even read the article :-)

in my native language, they both share same word - disk (hard disk, compact disk), though floppy disk had it's own word