The Vault writes a history of all of its activity to a file called _BACKUP.CTL and places it on the hard disk in the root directory or folder being backed up. It supplies The Vault with the date and label information necessary to automate the incremental backup process. In addition, a file called _BACKUP.HST provides the user with a helpful record of the files which have been backed up and a means of locating a specific file in the backup set. One can search for a file name in the list of files and thereby find the disk and folder on which it has been backed up.
_BACKUP.HST contains a four-line header followed by a list of files. The header contains the date of the most recent backup, the date of the last full backup, the last output folder used, and the last incremental disk used. The list of files contains the full pathname of each file that has been backed up, the creation date of the original file, and the disk and folder numbers of the backup copy. _BACKUP.CTL is the same, except that it does not list the files which were backed up.
Each time an incremental backup is done, the backup history is modified. The current date, output folder, and incremental disk are written in the header, and the names of all files backed up are appended to the end of the list of files.
When a new full backup is performed, the old _BACKUP.HST file is renamed to _OBACKUP.HST before the new _BACKUP.HST file is written. Thus, the old history is preserved in case the new backup fails.