To isolate and fix the issue, try the following the steps:
1. Perform a disk-based backup to eliminate the tape device issue and if it works on disk then try the below steps
2. Clean the tape drive.
3. Run an inventory and quick erase on the tape media.
4. Insert a different tape, preferably a brand new piece of media.
5. Check the properties on the tape drive(s) in Windows Device Manager to ensure that the latest Symantec Tape Device Drivers are installed. Install the latest drivers for the tape drive if available. For information about downloading and installing the latest Symantec drivers, please refer to the following link
http://www.symantec.com and then search for "Downloads".