From what I recall from the time I still used MS-DOS, even if you only have one floppy disk drive, it's accessible as both A: and B: and it prompts you to insert the other floppy disk when you change the drive letter. That is, it's "virtualizing" two floppy disk drives using a single physical one. That explains why the first hard disk drive was always C: even when you only had one floppy disk drive (and of course you had at least one floppy disk drive, how could you use a computer without one?)

Which is very convenient (and necessary for backwards compatibility) because programs could be (and were) written to assume they're in B: (and the DOS disk is in A:), or that they're in A: and the user's data is in B:, or a two-disk program is in both. All of that just worked (tm) even with a single drive, as long as the program didn't try to interleave accesses to both drives (I think it would still work because DOS was single-process, single-thread, it would just annoy the user!)