DOS and Windows keep a so-called archive bit for each file, which acts like a warning flag. If a file is saved during a backup, the archive bit is cleared, the flag is put down, so to speak. As soon as you make a change to this file on the hard disk, the archive bit is set again. The operating system puts up a red flag which signals: Attention – this file was changed and should be backed up again!

Windows automatically sets the archive bit on every change to a file, complete overwriting (Save as …) and file creation.

Backup strategies and the archive attribute
 

The archive bit is treated differently depending on the backup strategy.

•At a copy backup, e.g. burning data to a CD/DVD for backup purposes or copying data to an external hard drive, the archive attribute usually remains unchanged.

•The full backup saves all files, no matter what their attributes are, and clears the archive bit.

•The differential backup saves all files that have changed since the last full backup and thus have their archive bit set. The archive bit remains set.

•The incremental backup saves all files that have changed since the last incremental or full backup and thus have their archive bit set. The archive bit is then cleared.

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