SureStore T20 takes less than 9GB

[oscar_71]

Have a Tape Unit HP SureStore T20. I Use tapes hp travan 20 GB data cartridge C4435A, that tells as following:

For use with 20GB/TR-5 drive.

I have 9Gb on my HDD, tapes are not capables to save it in one unit.
Don`t know where the problem could be.
It`s over Hp Netserver E60 with Server 2000.
I Use Tapeware 6.20 SP3C.

What should I look first? Drivers, Other tapes, Other Software?

Thanks in advance