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”.) "

en.m.wikipedia.org/wiki/Disk_storage