In my case, the configuration was Compaq AIT 35/70 drive in Compaq ML350 server running W2k, SP3 and Backup Exec 8.6, Build 3878. The drive just started getting errors after successfully working for almost a year. Contacted Compaq Support and tried replacing drive twice. Downloaded updated drivers from Veritas site. Ran Compaq Tape Diagnotics and updated Compaq drivers with most current SupportPaq. Did reinstall and uninstall/reinstall of BENT 8.6. This got drive working for almost a month with no issue.
Had repeat of errors in System log and went to device tab of BENT 8.6, selected drive in Device Management and rightclicked to get properties. Selected Configuration tab and deselected checkbox to "Enable device for Backup Exec". Applied and selected box again, applied. After I restarted server, the error went away.
This may be due the fact that the SCSI driver on the system is out of date. See Veritas TechNote ID: 231918 link below.
for a troubleshooting approach.