We had similar problem every so often. Our 3494 is over 14 years old and
many of the upper middle cartridge slots sags a little, especially
column C.  The laser sometimes has a hard time reading the barcodes of
those slots. 



How does the 3494 determine which 'media type', e.g. 3590 J or K, a cart
is? Our 3494 is starting to eject tapes as they are inserted complaining
about unknown media type. These are tapes that have been in use for a
number of years and are being reinserted into the 3494 after coming back
from the offsite vault. Most of the time they are being correctly
recognized simply by reinserting them into the library another time. Our
non-IBM CE doesn't have a clue.

David