>We have a problem with our 3494 Tape robot with 3590 drives...

Be sure to have the latest level of the 3494 device driver (aka LMCP, aka 
atldd).
Older levels are prone to latching in a state where they falsely believe that
the 3494 library is offline when it isn't.  You can obtain this device driver
via authenticated FTP access.  Get information about this by going to
index.storsys.ibm.com and first retrieving the devdrvr/drivers.access file.

You should consider running some kind of daemon, based upon the mtlib command
or a program issuing ioctl MTIOCLQ calls, to periodically check the state of
the 3494 library and your connection to it.  You want to avoid the situation of
where the 3494 might lapse into an Intervention Required state and no one
realize it for some time.  If you want a sample C program issuing such a call
and interpreting possible error conditions, send me mail directly.
        Richard Sims, Boston University OIT