For the terminally curious, Apple has a knowledgebase article: What’s the difference between a “disc” and a “disk”?
Short story: Discs are optical, disks are magnetic. But you knew that already, right? And:
Although both discs and disks are circular, disks are usually sealed inside a metal or plastic casing (often, a disk and its enclosing mechanism are collectively known as a “hard drive”).
Absent from the article is any mention of how it came to pass that Apple gets to speak so authoritatively on the subject. Granted, this seems to be standard tech lore, but it’s weird to see the KB regarding itself as if a dictionary.