We have an HP tape loader partitioned for 4 daily backups (one tape each) and one weekly backup (3 tapes). The tapes are LTO-1 with compression and recently the tapes seem to be selectively choosing when to compress the data. Our RAID data to be backed up is 557GB so with the LTO-1 compression, we should be able to back up 600GB, but around 446GB, the third tape is filled up and we get the prompt to "Please insert overwritable media into the robotic library using the import command." and Event ID 58064 is written to the event log. If compression was not working at all, then we should not have been able to write past ~300GB and if the third tape was not taking compression then it should have triggered at 500GB and not 446. I tried changing the Advanced setting from backup following Junction points to Symbolic links but that made no improvement.
Installed update are SP5, Hot fix 358179, 141388 and 155482. Server OS is 2003 R2 SP2.