You didn't specify whether these were 3592 generation 1 or 2 drives. If gen 1, the 3592C you have set is the correct devclass specification.
I would check that your tape device driver is the correct type, and is a recent vintage, to avoid any vagaries in passing device commands from TSM to the drives. Defects in drivers are not unknown. Likewise for tape drive microcode/firmware. See if you can find a utility to inspect the drive settings, to verify what they are. In AIX, the lsattr command can be used to verify if compression is in effect; I don't know what might be used in Windows. Richard Sims
