It's the same thing. The difference is origin. Disk is American. Disc is British. Usually the only time "disc" is used in the US, is to refer to something round. A CD could go either way, depending on the writer.

Perhaps it's just a leftover marketing motif?

"The spelling disk and disc are used interchangeably except where trademarks preclude one usage, e.g., the Compact Disc logo. The choice of a particular form is frequently historical, as in IBM's usage of the disk form beginning in 1956 with the "IBM 350 disk storage unit". "

https://en.m.wikipedia.org/wiki/Disk_storage

I’ve always viewed it as the Disk contains the Disc. IOW, the floppy has the magnetic disc in it. The optical disc is the disc without the Disk.

Probably completely wrong etymologically, but semantically it’s fun.

Disk but with a soft k, like in kif

Disc seems to be anything with a round and flat outer appearance, where disk seems to refer to any other storage media

I think they're the same word, though.

Same word. Different dialects.

There’s been some movement over time but in general disk was used for pc because you had Hard Disk Drives. Then their counterpart the floppy diskette (disks).

Disc as a term was used for media like compact discs and subsequently digital video discs, etc. and then pc components allowing them to be read and then written to did exist for PC’s and, as such, had the disc moniker. But that’s because they were already “discs” branding wise.

USB thumb drives, being created as portable removable media for pc’s were a kind of solid state disk and so they use the k. Even NVME, being primarily storage for computing devices, can also colloquially be called “disks” but more and more people just refer to them as drives and I suspect those who refer to them as disks may do so out of older computer hardware habits and that utilities (fdisk, df, etc)call any such media a “disk”.