Bootable backups allow you to instantly return to a previously saved state on your computer by booting directly from your backup media. All of your files, applications, and preferences are preserved exactly as they were when the backup was taken. Tools like Carbon Copy Cloner, and SuperDuper make cloning your hard drive to a bootable backup easy, and save time by transferring the files that have changed on a regular schedule. Unfortunately the benefits of a bootable backup are small compared to other backup solutions, and their weaknesses require bootable backups to always be part of a more robust backup strategy.

Time

John Gruber, Shawn Blanc, Frank Chimero, Antonio Carusone, Patrick Rhone, and Chris Bowler all use bootable backups as a method to get back to work quickly when a hard drive failure or data corruption prevents them from booting from their primary hard drive. They know that once their work is complete they will have to repair the issue and either restore their bootable backup onto their primary drive or rebuild their computer. If they are not able to repair, or restore their computer on their own a bootable backup is a fast affordable alternative to on-call tech support.

Bootable backups don’t save time, they allow you to put off repairing your computer until it is more convenient, or until help is available. Even with a bootable backup a repair needs to take place otherwise you would be working off your backup disk until it fails and you have no bootable backup left to rely on.

Data

Bootable backups should never be the only way of preserving your data. By their very nature a bootable backup is a clone of your computer’s hard drive, and a clone can only contain a single revision of your computer’s files. If a file becomes corrupt without your knowledge and inadvertently get copied to your bootable backup you now have two copies of the damaged file and no backup. If a problem on your computers becomes apparent only after it have been copied to your bootable backup you now have two problems, one on your primary hard drive and one on your bootable backup. If you accidentally delete a file, save over a file, or change a file for the worse you can’t undo your mistake once it has been copied to your a bootable backup.

Bootable backups don’t protect your data from yourself or unforeseen computer issues. They merely provide you with the means to go back to a earlier point of time even if that point of time is incomplete or corrupt. A good backup strategy would include a backup system that maintains revisions of your files. Restoring the file you lost saves far more time then booting to the file you don’t have. Never trust your data to a bootable backup alone.

Confusion

Filesystems are confusing on their own, but duplicate filesystems mounted in tandem are twice as likely to cause users to save their data in the wrong place. Just ask Adam Engst a Macintosh expert I deeply respect, who accidentally saved some photos onto his bootable backup and then inadvertently removed them with a nightly clone. This sort of mistake is especially easy to make because a bootable backups is almost indistinguishable from your primary hard drive, and cloning software works by blindly replacing files on the bootable backup with the files on the primary drive. Since Adam’s files never existed on the primary drive they were not only removed from his bootable backup but never picked up by any of his other backup technologies that only look for changes on his primary drive. Adam was able to recover his pictures thanks to a an additional copy he hadn’t expected, but if that copy hadn’t existed Adam’s bootable backup would have been the cause of his data loss.

To eliminate the confusion caused by bootable backups a bootable backup should never be mounted at the same time as the primary drive while the computer is in use. Adam learned his lesson by scripting his bootable backup to mount automatically during cloning, and unmount when the clone is complete. Every day users might not have the skills required to solve this problem, and manually mounting and unmounting the backup drive is more likely to lead to forgetfulness than a nightly bootable backup.

I can’t recommend bootable backups for any of my users due the limited advantages, and confusion they provide. An hourly backup like Time Machine that maintains revisions that are recoverable by the user is far more useful especially when you consider how easy it is to rebuild your machine from a Time Machine backup. I understand why knowledgeable users insist on having a bootable backup, but a failure that requires a Nuke N Pave only takes me an hour to complete, and gives me a clean rebuild no bootable backup can provide.