The other possibility is that since the 2108 was replaced, there may
be a small chance the /dev/rmtX mapping no longer matches what the
previously assigned rmtX-to-element ID mapping was.

Which conceivably could produce this sort of error, if TSM and
the library disagreed about what logical/physical drive matched up?

What I'd suggest is:

tsm> q drive f=d

Then note each rmtX device name and what element ID it corresponds to.
(in TSM)

Then do some tracing to figure out which element ID each rmtX device
actually physically corresponds to.

Then you'll soon see if the TSM setup and the probed devices in AIX
matches or not.

If they don't match, update the TSM drive entries for rmtX and element ID
ASAP.

Should normally take only about 5-10 minutes of work to figure out the
mapping from scratch. Keep good notes and issue a whole bunch of commands
to assist in the logical-to-physical tracing.

I just finished fixing all this after no end of fun with the 3584 setup
and mismatched rmtX/element IDs.

-Dan