When I get "Unable to position media", it is either a duff tape or, more likely, a faulty tape drive.
I would suggest our normal procedure (if you haven't done this already)
1) clean tape drive thoroughly
2) get a new tape (out of the packet) and use this to do your next backup. If you are able, you could do a small test backup, but this isn't always conclusive as a small backup doesn't allow the tape drive to get going properly, and it is likely that the errors occur when your tape drive has been going for a little while.
3) if that doesn't work, I suggest a new tape drive.
Hope this helps.