[dwb]

I read somewhere a long time ago (I forget where) that it was (or tended to be) "disc" where the format came from the audio world, and "disk" otherwise. So, "Compact Disc", but "hard disk". No idea if this is rubbish or not but maybe it jogs someone's memory.

[valbaca]

The way I was taught was that "disk" is when you don't touch it directly (disk drive, floppy disk) and "disc" is when you can (compact disc, frisbee disc).