disk vs disc?

[thaichicken]

Hello all,

I was writing something the other day about CDs ("hard copy music") versus data stored in a computer/ipod/etc, and I went to write out "disk" and realized I didn't know whether it ends in a c or a k. Disc and disk both look right to me, and google has been no help.

Is this just a matter of personal choice, or is there some little-known distinction? Or, alternatively, is one of them one of those words where if you misspell it long enough, the second spelling is adopted?

Thanks!