In modern parlance, this has been my working understanding too:
But yeah since computing came along disk has also been used more for magnetic media (hard disk) while disc has been used more for optical media (compact disc).
Optical:
- compact disc
- laser disc
Magnetic:
- 3.5" diskette
- 800GB ard disk drive
...and just to point out there is some disagreement
Magneto-Optical , such as Sony MiniDisc, is sometimes referred to Disc for its optical properties and sometimes as a MO Disk for its magnetic properties.