This is the explanation of the ASSIGN command. It is EXTERNAL.

Assign redirects requests for disk operations from one drive
to another. It is helpful with programs that can only read 
one or two certain drives. Using the assign command you assign
the unreadable drive to the readable one.

To get a list of all the assignments currently in use type:
     assign /status

Example:
       To use drive C with a program that can only read and
       write in drives A and B you would use the following
       assign statement to solve the problem:
            assign a=c b=c