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 hard 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.