Most common usage I've seen: Disk refers to a medium of storage. Disc refers to a shape. Hard disk drive (means of storage) / Compact disc (referring to the shape of a CD). I've also seen usage that disk is magnetic storage (floppy disk, hard disk, ZIP disk) and disc is for optical storage (CD, DVD, Bluray).
Strangely, you never hear of disk/disc being used for electronic means of storage like for USB flash drives or SSDs.