We are currently running a GFS backup scheme and my full backup runs friday night. When I use a blank tape with compression to 40GB it only backs up between 28-31% of my 30GB of data and then asks for a second tape. This data should fit easily on and I have no idea why it's doing that. The ONLY thing I could possibly think of is that I do a long erase on some of these tapes to erase old data and re-use them for the full backup as I have no new tapes.