IBM 5150 Manual To Operations page 105

If You Have Two Drives
1
Make sure DOS is ready and A> is displayed.
2
Insert your DOS Diskette into drive A: if it is not already
there.
3
Insert the back-up diskette into drive B:.
4
Type this:
format b:
and press the Enter key.
5
You will see this message:
Insert new diskette for drive B:
and strike any key when ready.
6
Press a key, because the back-up diskette should already
be in drive B: (step 3).
DOS 3-31