"That's why D was typically the CD-ROM:"

We used to set our machines so the CD-ROM was always drive L. This way we always had 'room' to add HDs so there was no gap in the alphabetical sequence. Drive D - data drive, E - swapfile, etc.

Test and external drives (being temporary) were assigned letters further down than L. Sticking reasonably rigidly to this nomenclature avoided stuff-up such as cloning an empty drive onto one with data on it (cloning was a frequent activity).

Incidentally, this rule applied to all machines, a laptop with HD would have C drive and L as the CD-ROM. Machines with multiple CD-ROMs would be assigned L, M and so on.