A data backup application checks validity of data being backed up during an incremental or a full backup. In case of the file backup, the full backup checks the validity of data during the backup, because in case of data corruption the user will not have any original data since no data has been backed up yet. In case of the incremental backup, only the new files or the files that have changed compared to the last backup are now backed up. All new and modified files can be checked for validity by comparing internal format of the user files against a corresponding file format standard. If corrupted files are detected among the modified files, the user is notified.