A new file backup system facilitates backup of a file while it is being updated. The file header includes a set of flags which synchronize the backup operation. After the backup operation has begun, if a portion of the file being backed up requires updating, the portion of the original file to be updated is maintained in the digital data processing system. The backup control program performing the backup backs up the file, substituting the original portions for portions that have been updated. Since the updated portions of the file are maintained during backup, the file can be accessed and updated while the backup operation is taking place.