For VMS systems, there are two DCL commands which must be done prior to entering IDL in order to access the tape drive. Issue the following two commands before starting IDL:
$ Allocate tapeDrive mt* where tapeDrive refers to the 'mu' device (example: 'mub0') and mt* refers to mt0 thru mt9, pick one. $ Mount/FOREIGN mt* this mounts the tape to the 'logical device' name mt* (again mt0 thru mt9).
Then enter IDL. Assuming one selected `mt0' as the logical device name for the tape drive in the allocation command, the valid rdtap command restoring the tape directory files would look like: IDL> rdtap, def_tapd(0), /dir