It sounds like I have the same problem. We even have COMPAQ and Veritas looking into it and they each blame the other. Here's my setup and what's happening.
1-Master
5- NT media servers
2- 2000 media servers
20 or so clients
All COMPAQs, 2 ML570(NT media servers, one a PDC), 8 DL380 G1(1 is master, 1 is media), 5 DL380 G2, 2 ML370 (1 media, SMS), 7000(media), 2 5500(1 media), some 2500 and 1600s (1 media).
The master and media are HBA(64bit, 33MHz) fibre connected to a 2 port MDR which connects to 2 SSL2020 with 2 AIT drives each.
The master has no problem backing up the clients or itself. The media servers on the other hand are always bringing DOWN the drives. The HBA cards are to firmware revision 3.82a1 and the tape drives and MDR are at the latest. The drivers on most are to 4.53a7 for the HBA and the tape drives are VERITAS 4.0.1381.1.
Not all are up to the latest because we were testing after each server and we got to one of the ML570s and it started DOWNING the drives again. Before we got to that one it seemed to be working after loading the right drivers.
The errors we get in event viewer as follows:
TL8(0) [2792] Unload drive 2 (device 1) failed in io_open: The media in the drive may have changed.
TL8(0) drive 2 (device 1) is being DOWNED, status: Unable to SCSI unload drive
Check integrity of the drive, drive path, and media
One of the test ones I did came back with all the same except on the first message it had :I/O bus has been reset
Yesterday in testing we found that if two media servers tried to go at the same time the one that started second would down a drive or 2.
We have already turned off Removable Storage service. Might be stuff I'm missing but that's a start. I hope some of this helps someone to start looking in the right place. I have been working on it for almost 2 months.