I also had the problem that after replacing a PBS and reinstalling the changer/tape library which contains two drives I could not restore the content of the tapes - while there were error messages stating that the drive could not be loaded...
seems the drive ids are reversed in your config, and the changer uses id 0 for Naped2 and id 1 for Naped1, which would explain the weird state, because
the part that looks at the changers sees the drive in the "correct" drive, but it's actually...