NT 4 Backup problems on a Notes server..

I'm having a problem with Disk Compression and it's driving me insaner..
I'm using a Sony SDT-9000 series tape backup drive on an NT 4 system running Notes as our mail server. I'm using DDS3 HP tapes (new). DC is enabled on the drive, and the Backup Exec (v.7) software is set to compress via Hardware, not software. I'm only getting 12Gb on my backups instead of 24. I called Sony and they said I "must be trying to backup files that are already compressed". So here's my question: What (if any) files are considered "compressed" in the C:\notes\data directory? Thanks in advance for any help.

p.s. Jumpers on the drive are correct, already checked that.