jmtx -f /dev/t950 status|load|unload [slot#] [drive#]
You must be user production to run. The "-f /dev/t950" is vestigial from the mtx command. The jmtx will talk to its configured tape_svc which determines the device. It is left in to be symetric with the mtx command and to show the users intention.
jmtx will still work with drives that have been taken offline via e.g.
driveonoff off 10
If you're going to work with some drive, it is usually best to take it offline first, so that you and SUMS don't interleave your use of it.