Definition: B: drive

The second floppy disk drive in the first CP/M and DOS/MS-DOS personal computers. Initially, two 5.25" drives were used (A: and B:), while some subsequent machines had one 3.5" A: drive and one 5.25" B: drive.

Hard Drives Became C:
When hard drives became available, it took the next letter and became the C: drive, which to this day is the primary storage drive letter in more than a billion Windows computers.

Windows assigns D:, E:, etc. to subsequent storage drives whether internal or external, but there are no more A: and B: drives. However, an external floppy drive plugged in via USB may assume the A: designation or the next letter following all storage drives.