IBM 5150 Manual To Operations page 143

If you had two drives
Original
Back-up
Diskette
Diskette
But with one drive
Original Diskette
pu t in when drive
A: is called for.
So you should:
Insert When
Original diskette "drive A:" message appears
Back-up diskette "drive B:" message appears
Let's assume that the file you want to copy is LETTER.
For this example, we are going to copy LETTER to
another diskette without changing its name.
DOS 3-69