Well, I have to admit, I'm at a loss as to why you're experiencing the bug. Perhaps if you run 'tar' under GDB you can see why it thinks the tar file is ustar format rather than GNU format. I expect that it's some unanticipated confusion between -r (which needs to guess the tar format from the existing tape) and -M (which allows for multiple tapes, some of which may be of different formats!).
One thing that strikes me is this one: sudo tar -rMf /dev/st0 /path/G* (tape 7, 8) Was tape 8 initialized before this tar -rM command was run the first time? It could be that you need to initialize the trailing tapes as well.
