Hi Jesper,
Run the C:\program files\veritas\backup exec\nt\tapeinst.exe
Remove all the veritas driver and have it delete any undetected drives.
Power down the server and tape unit.
Power back up and let the OS load the windows drivers.
Run the tapeinst.exe and re-install veritas drivers and reboot.
If that doesnt fix it then the SCSI drivers may have changed in your upgrade. Update the firmware on the tape device, SCSI card and update scsi drivers.
I have had luck flashing the tape device with Compaq firmware. Also you may want to check to see if the library or tape unit can emulate another drive/library type
Hope this helps
---cheers
shirmal