When PROBKUP writes a block of data to the backup media, it calculates a CRC code based on the contents of the block and stores it with the block. When restoring, PROREST re-examines the contents of the block and verifies that they are consistent with the accompanying CRC code. If the block contents are not consistent with the CRC code, the backup block is corrupted.

If the backup includes error-correction blocks, PROREST automatically uses the information in those blocks to recover the corrupted block. If the backup does not include error-correction blocks, PROREST cannot recover the corrupted block when you restore the database.