I looked in the archives and found nada on this, as in nada whole lot. Perhaps “disc” was introduced by Sony as a marketing ploy when compact discs first came out, a more hip spelling that young people would find “cool.” I’m just guessing here. Anyone have any pearls of wisdom on this topic?
Maybe because the “disk” in floppy disk is short for diskette?
That’s just my WAG, by the way.
I had always thought that disc was the UK spelling and disk the US and Canadian spelling for the same item.
I think missbunny has it. Disk is short for diskette, while disc is short for discophile, as in a record. No?
Disc is the UK spelling, but I can’t seem to find any two sources that agree on where the CD was invented (apparently by Phillips in collaboration with Sony).
disc and disco have any relations?
clueless
Disco is short for the French Discotheque, which I think might be a play on words (Library is Bibliotheque), but IANAFrenchman.
My WAG: the Disc spelling is used when the media is actually ROUND. Granted Floppy disks are round inside a square plastic container, but you get my drift.
Both “disk” and “disc” are correct spellings in American English, and (peers into ancient OED) as of the late 19th century, British English as well.
It’s been that way since long, long before Sony and Phillips decided to name their standard in the 80s. Presumably someone at one of those companies just preferred the spelling “disc.”
(I think the spelling confusion comes because the word comes to us from Greek via Latin. As a Greek word, you’d spell it disk; as a Latin word, you’d spell it disc, e.g. discus. See also Herakles vs. Hercules)
We know him as Heracles. 
I saw somewhere (yeah, great cite, I know) that some group or another was trying to standardize “disk” as representing magnetic media and “disc” as representing optical media. I know I saw this sometime relatively recently. I’ll try to track down where I found it.
I, too, thought that “disc” referred to optical media in common usage now. Shows what I know 
While I found plenty of counter-examples in use, I did find this on a University of Nebraska site:
Full site is at http://www.ianr.unl.edu/pubs/consumered/nf457.htm
Disc is British and is/was used in Britain’s former colonies. Disk is U.S. Disk is taking over because Micro$oft is U.S., as is Apple, Sun and all the rest. Apple ditching the U.K. OS hasn’t helped, either (color from colour, etc.). A few style books outside the U.S. say to use disk only in the context of computers and disc everywhere else. This is a copout. The real reason is to cut down editing on wire copy.
For what it’s worth, according to a computer science teacher in my high school (this is going back 20 years now) when I asked, “disk” was the spelling preferred by IBM, who at the time pretty much could do anything they want. Had nothing whatsoever to do with diskettes, as the “disk” spelling came about long before diskettes were invented.