SCSI DAT40 drive writes more data to an DDS4 tape

[Niels Bode]

Hi,
i'am using ArcServe to Backup some images to DDS4 tapes using the HP DAT40i. When i backup about 17.000 MB of Data, ArcServe shows the correct amount (the estimate) but writes far more data to the drive. Currently, the "MB proccsed" shows 28.000 MB and is still counting. Verification in ArcServe is off, and the data is allready very compressed. I would understand it when their would be less data transfered to the drive (due to the internal compression), but MORE? Even with some "bad" sectors on the tape, this could not push the physical tape size to almost 30GB when the data is only 17GB.

Where is my mistake?
Thanks,
Niels