…as in the thing you store computer files on.
Even the websites of companies that deal in removable media seem to mix and match the two spellings, sometimes in the same sentence.
So which is it, and why? Or is it the subject for a great debate?
per AP:
YSMV (Your style manual may vary.)
I assume this puts CD in the “disc” category.
Magnetic media (floppies and hard drives) are disks.
Optical media (CD,DVD,LD) are discs.
There aren’t any hard-and-fast rules here, only conventions; ‘disk’ is the American spelling, ‘disc’ is the English one - both terms are completely valid and equivalent in their respective domains.
The problem is that the term then gets embedded into a meta-term and (usually)sticks whichever way it started - floppies and hard drives are disks, CDs and DVDs are discs - no rhyme or reason for it, just established usage.
FWIW, Seagate, alone among hard drive manufacturers, spells it disc.
Well, in the case of floppy disks, I know that “diskette” just looks and feels better than “discette”. For one thing, the pronunciation of the first spelling is more obvious than the second.
CDs, DVDs, etc have never been referred to as “diskettes”.
I don’t see how that proves me wrong; all it demonstrates is that conventions are not immutable or universal, but I never claimed any such things.
Thanks for that. I’m translating the user manual for a digital surveillance system into English, so I think I’ll go along with Pulykamell and Mangetout and use “Disk” for magnetic storage media and “Disc” for any other (non-botanical) platter-shaped accoutrements.
Oops, and Neutron Star 
cf program and programme
The post to which I last replied seems to have gone missing… hmmm
Well if it was a hallucination, I had it too…
Now people are going to wonder about my reference to non-botanical optical media.
An old rule of thumb is that, in the U. S. at least, one generally uses the spelling “disc” when referring to an object which is in and of itself known as “a disc”; hence, “he suffered a slipped disc and had back pain ever after”, but use the spelling “disk” when referring specifically to shape of an object: “the recipe calls for forming the meat into small disks”.
In the late 40s UFOs were often referred to as “flying discs” or “flying disks” as well as flying saucers. This custom may have died out because newspaper editors could not arrive at a consensus as to which spelling to use.
The convention would seem to call for referring to the disk- shaped computer devices as “discs”.
The same is true on this side of the Atlantic (only in reverse); we use the spelling ‘disk’ when referring to an object conventionally named as such (floppy disk), but use the spelling ‘disc’ when referring to the shape.
What about DISCK?