Cause
This error usually occurs because of a Bad Block on the storage media .
A Bad Block is a block (usually the size of a sector) that cannot reliably hold data because of a media flaw or damaged format markings.
This can be verified by running a verify job. The verify ensures that the preceding backup operation was successful. It will count the amount of file marks that the backup reported of the amount it just wrote to the tape. Running a separate verify job on a tape will ensure that the file marks are valid on the tape