From Scott Hirsch: Guess what happens when you try to access a labeled tape on an LDEV# > 255?

Answer:

SYSTEM ABORT 2559 FROM SUBSYSTEM 99

COMPATIBILITY MODE SUDDENDEATH 8606

SYSTEM HALT 7, $09FF

It seems MPE/iX's labeled tape facility is STILL in compatibility mode and HP has no plans to convert labeled tape handling to Native Mode.

The only work around: make sure all tape drive that could EVER be used with labeled tapes have an LDEV <= 255.