It means the tape or hardware has gone bad.

This is the probable sequence:

1. a BAK file is generated and a CRC code is generated

2. the BAK file is written to tape

3. the tape is re-read and a CRC code is generate

The CRC codes from step 1 and 3 don't match. Boom! Time to replace the tape or the tape drive or the computer attached to the tape drive.