IBM 5150 Manual To Operations page 84

Changing the Default Drive
You can
change
the default drive. To do this, just type
the new drive designator following the A> prompt, and
press Enter.
For example, to make drive B: the default drive, type:
b:
and press the Enter key. Here's what this looks like on
the screen:
A>b:
The"ci.rsor is here.
Note that now the prompt is B>-just what we wanted.
Now DOS will look at the diskette in drive B: whenever
you specify a file without the drive specifier.
You can change back by typing a:. Here's the screen
now.
8>a:
Try this too.
DOS 3-10