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 generated2. the BAK file is written to tape3. 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.
We walk in the dark places no others will enterWe stand on the bridge and no one may pass