Hello,
I have had issues with my ML350 20/40 unit, and am still not sure where they came from, however, I spoke to HP, and they told me to go to Hp.com and search for L&TT (Library and tape tools). I downloaded this program, executed and installed on the server with the tape drive. The file name is hp-ltt34-win , assuming you are running W2k server.
It tests the drive and tapes and it may help solve the issue.
I use Veritas Back-up Exec 8.5, and one thing I found was that somehow, I was using the Microsoft tape drive drivers instead of the Veritas driver, so somewhere in the help screen as I recall, there is a link to install the Veritas drivers.
Good Luck
BFCLEM