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 Disk and sometimes as a MO Disk.