You're concentrating your attention on ADSM, but what about the 3494?  Run
some 'mtlib' commands to check the status of the drives and the tapes in
question (particularly for unexpected category code values); check the 3494
control panel for Int Req and error conditions and drive availability
settings; check the tape drive panels for their conditions (like unload
failure).  At the 3494 control panel, look up the tape in the Library Manager
database.  Do an ADSM 'show libr'.  Check the drive status from the point of
view of the operating system.  I'd say that there's something amiss in the
3494: ADSM wants the tape to be mounted, but for some reason the Library
Manager can't do it.

The Mountwait parameter isn't all you might hope for.  ADSM tends to
wait indefinitely for a mount during a problem condition, ostensibly
because the mount request has not yet been accepted by the Library
Manager for the MountWait timeout clock to start ticking.

     Richard Sims, BU