A recurring problem I have often encountered over the last decade when setting up Backup Exec for backup in Windows environments, has been that backup jobs have failed with a unknown error. (An unknown error occurred)
This problem has sometimes been fixed by upgrading the tape drivers from Symantecs website, or upgrading to the latest version of Backup Exec (if available).
When this error also occured on a new system with Windows 2009 R2 and Backup Exec 2010 (version 13) with the latest drivers installed, I had to do some hard core reasearch to find a solution.
I finally found this link on Symantec support web site: http://seer.entsupport.symantec.com/docs/336636.htm. It describes a setting in the windows registry for adjusting the number of retries accessing the tape drive. I set the value to 250 (hex fa).
After setting this value, the backup has been running without any problems.