IBM 5150 Manual To Operations page 145

If you do see the message, do this:
8A
Remove the back-up diske,tte from the drive.
8B
Insert the original diskette into the drive.
8C
Press a key.
8D
Now go back to step 4.
9
Hint: It's easier to remember which diskette to insert if
you think "B: is for back-up". Insert the back-up
diskette when the drive B: message appears.
When the copy has been made, this message is displayed:
A>
1 File{s) copied
10
11
Remove the back-up diskette from the drive and label
it with a felt-tip pen.
Now you can put both diskettes away-LETTER has
been copied to the second diskette.
For You to
Try
Select a file on your DOS Diskette to copy to
another diskette-how about DONKEY.BAS? Keep
the name the same (unless the diskette you'll put the
copy on already has a file called DONKEY.BAS).
DOS 3-71