I have purchased new 12g/24g tapes to replace old tapes that are identical brand (HP) and type. The new tapes stop backing up at 15 gigs and ask for another tape. The old tapes continue on to backup the entire 18 gigs without needing another tape. I understand the 24 gig capacity is a maximum compressed capacity that is rarely achieved, but the tapes should be consistant. I have also tried an alternate brand (TDK) with the same result. Any ideas?