They both exist in different areas of the English language.

So, in fact, you are't being incorrect when you say grey for gray, your just naming off the wrong thing.

:D

I usually refer to digital ones as discs (music CDs) and something like an 8 1/2 floppy as disks. I'm not exactly sure where I picked that up from. =\

Both are on the same definition...

disk also disc Pronunciation Key (dsk)
n.

1. A thin, flat, circular object or plate.
2. Something resembling such an object: The moon's disk was reflected in the pond.
3.
1. The disk used in a disc brake.
2. A disk used on a disk harrow.
4. A round, flattened, platelike structure in an animal, such as an intervertebral disk.
5. Botany. The enlarged area bearing numerous tiny flowers, as in the flower head of composite plants, such as the daisy. Also called discus.
6. Computer Science.
1. A magnetic disk, such as a floppy disk or hard disk.
2. The data stored on such a disk: read the disk that came with the manual.
7. An optical disk, especially a compact disk. See Usage Note at compact disk.
8. A phonograph record.
9. A circular grid in a phototypesetting machine.

I belive Disc refers to optical storage media, while disk refers to magnetic based storage...

Disc: CD-Rom, DVD-Rom...

Disk: Floppy Disk, Hard Drive Disk...