PROBLEM WITH TAPE HARDWARE COMPRESSION

Hi!!! I've got an HP SURESTORE DAT 40 on my SUSE LINUX ENTERPRISE SERVER 8 BOX SP3. The tapes I use are marked 20/40GBYTE. But I can only make backup up to 20G, whereas on my ALPHA server with tru64 unix, I can save up to near 40 Gbytes using the dev file "/dev/rmt0h" (for tru64 V 4F) or "/dev/tape/tape0_d1" (for tru64 V 5.1B). I try to enable compression using the command "mt -f /dev/st0 datcompression 10", this command achieved succefully but nothing. I try to use the /dev/st0m or /dev/st0a, but these devices files seem not to be associated to physical device. So how can I perform backup with hardware compression? Please help