Ok, this is a shot in the dark.  I don't normally respond to questions here,
and I didn't see the first part of this post so please excuse me if this
does not pertain to you.

You wouldn't happen to have upgraded your drives would you?  Say from 3490
to 3590 as an example.  I have seen this happen when trying to migrate a
storage pool onto a different hardware.  It is trying to mount a 3490 volume
onto a 3590 drive.  Should work right?  Well it has a problem with it, and
you get a server mount not possible.

This would also explain why the migrate works.  If the new tapes are 3590
then it will mount just fine.  The direct to tape may be failing because it
is trying to mount a filling 3490 volume.

Just a thought.

Gary