How did it come to pass that we speak of disc brakes and floppy, or hard, disks?
I think “disc” came first (think “discus”), and “disk” is a shortening of “diskette” which is the original term for a computer disk. Perhaps someone else feels like researching this more thoroughly 
-Andrew L
I’ve wondered this often. I usually use “disk” four, well, disk-shaped objects, and “disc” for anything such disk-like object that contains information, except floppies, which I just call floppies and avoid breaking my own rule. 
I’ve never seen anything elucidate an obvious difference. Check Merriam-Webster’s comments and see if you can figure anything out.
IBM preferred “disc,” for whatever reasons.
ok, how does the number “four” make an appearance in that post? Hmm?!? Who put it there? ::taps foot impatiently:: :dubious:
The OED says:
The earliest use of the word (from 1884) is spelled “disk” and means the “face” of the moon or a planet. The earliest use of “disc” dates from 1727, but in the example “disk” is also given, so it looks like the two were just varient spellings.
It was probably a case of people using both spellings throughout; not all words have a single correct spelling, and disc/disk is one with multiple correct spellings.
Obvious, “diskette” came from “disk” and not the other way around.
All I know is that the hard disk of a computer actually is (or at least used to be) a disk, much like a old record. Of course, that doesn’t say anything about the difference in spelling…
FWIW, I have always heard that (as regards technology) “disk” refers to magnetic media and “disc” refers to optical. So, it’s floppy disk and compact disc.
And, just as an anecdote, at my first job out of college, our operations manuals all referred to “disquettes”. 
I’ve always heard the terms used the same way tanstaafl has just described, as far as computers are concerned.