[keitmo]

On systems with a single floppy, drives A: and B: were two logical drives mapped to the same physical drive. This enabled you to (tediously) copy files from one diskette to another.

[dmurray]

I don't recall this, and I do recall running something like "diskcopy A: A:" to do that operation.

[pxx]

phantom drive B is explicitly mentioned in https://en.wikipedia.org/wiki/Drive_letter_assignment#Order_...

the linked source checks out. diskcopy will also do this for you if you give it source = dest.