I have an extenal HP DAT 160 USB.
The system i use is a Windows Server 2003, i launch a backup over the device with NTBackup.
I cannot store in the dat more than 80 GB, that is the hardware compression seems not to work.
You need to make sure your system is using hardware compression.
NTBackup had a bug in the early W2003 editions that disabled hardware compression by default. Microsoft has a support note and hotfix for this behavior.