MSCP      Disk
                        +----------+    Unit      Type
                        |          +----  0 ----> RC25  (removable)
        +-------+       | First    +----  1 ----> RC25  (fixed)
        |       |       |          +----  2 ----> RA80
        | D     |Port 0 | UDA      +----  3 ----> RA80
        | U     +-------+          +----
        |       |       | Con-     |
        | H     |       | troller  |
To user | a     |       +----------+       
<-------| n     |       +----------+
program | d     |       |          +---- 20 ----> RC25  (removable)
        | l     |       | Second   +---- 21 ----> RC25  (fixed)
        | e     |       |          +----
        | r     |Port 1 | UDA      +----
        |       +-------+          +----
        |       |       | Cont-    +----
        +-------+       | roller   +----
                        |          +----
                        +----------+