Coincidentally, we had the same error this week, too. I had updated the drivers
on our Dell PowerVault 132T tape drives to the Dell drivers. Not that I selected
these drivers, they were installed via the Internet. The drives would go off-line,
also, and wouldn't even do an inventory.
Symantec support spotted the issue immediately as a driver issue and pointed
me to the driver update link from Symantec and, after downloading and installing
the new Symantec drivers (Jan. 2008 drivers), all was back as before. All told,
I spent less than a half hour on line with Symantec.
I'd check the tape drive drivers first. Good luck!