Using more than one backup program

Ata Amiri asked if using two separate backup program will interfere with incremental backups.

An incremental backup copies only files that have been created or changed since the last backup.

Using two different backup programs on a regular, daily basis provides extra protection, especially if you're backing up to different media.

The danger of using two programs for incremental backup lies in each program skipping files that were backed up by the other one, because these files show up as already backed up. This would result in two incomplete backups.

A few years back, that was a serious concern. At one time, virtually all backup programs used a file attribute called the archive bit to determine which files needed to be incrementally backed up. When Windows (or DOS before it) saved a file, it turned the archive bit on. When a backup program copied the file, it turned it off. With this approach, you would never incrementally backup the same unchanged file twice.

Most modern backup programs ignore the archive bit, and use internal databases to determine what files to backup. That way, multiple backups are not a problem.

For instance, I daily use Mozy to backup to the cloud, and Cobian Backup for local protection. They both get all of the files in their overlapping backup sets. (Yes, I know that Mozy can backup locally, but I feel more secure using two different programs.)

If you want to make sure that your two backup programs of choice will work together, you can easily test them. Run a full backup with one of them, and then with the other. Change one existing file, and create one new file. Then run incremental backups with both programs. If both caught the new and changed files, you're good to go.

Contributing Editor Lincoln Spector writes about technology and cinema. Email your tech questions to him at answer@pcworld.com, or post them to a community of helpful folks on the PCW Answer Line forum.

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