so you have a tape drive [with compression enabled] that by default can backup 20gig, you backup is 37gig, this would appear to be normal. ARCserve will need a second tape for this job. If any of the 17gig [stuff over 20gig] is compressed already, i.e. zip files, databases etc.. then hardware compression is not not going to compress them anymore. Using software compression while you have a tape drive with hardware compression is daft, use one or the other [i would choose hardware] not both.