Drive Existence

                         Detecting Drive Existence

       1.  Save current drive  (INT 21/19)
       2.  Set current drive  (INT 21/0E)
       3.  Get current drive  (INT 21/19)
       4.  If current drive == drive requested
              then drive exists
              else drive doesn't exist
       5.  Reset original drive  (INT 21/0E)

       - no critical error is generated if invalid drive is selected