Comment by cesarb

> A was the first floppy drive, B the (typically absent) second floppy drive

As another commenter mentioned, when you didn't have a second floppy drive, A: and B: mapped to two floppy disks in the same floppy drive, with DOS pausing and asking you to insert the other floppy disk when necessary. Which explains why, even on single-floppy computers, the hard disk was at C: and not B: (and since so much software ended up expecting it, the convention continued even on computers without any floppy disk drive).