1. Hardware compression? I remember SunOS has numbers tacked-onto the dev names to control this, but I've seen inconsistency between the device numbers and whatever the lights on the drive say it's doing for compression. I trust the lights. Lights is all Spock had.

2. Rather than try to read your data file sequentially after reading the header file, have you tried walking the tape forward with "mt fsf" to see if you can get to the data file? I would try it before rewriting the header file first just to see if it does what you expect.