How to Encrypt Time Machine Backup Data

Would you like to secure your Time Machine backup data so other people wouldn’t be able to view your files in case your backup drive fell into the wrong hands? Apple does not provide a built-in way to encrypt your Time Machine backup files, but it can be done. A clever individual named Jay has figured out how to do it and has the best documentation that I could find on how to set up encrypted Time Machine backups. Thanks Jay.

[Update, Fall 2011: Mac OS X Lion’s version of Time Machine now includes a built-in way to encrypt a backup hard drive and its contents. Turning on encryption is now as easy as checking a box in Time Machine Preferences. If your backup drive is empty then the drive can be encrypted in about a minute. If the backup drive already has data on it then it can take many hours to encrypt the entire drive.]

In previous Tech Tips, I’ve written about how useful Time Machine can be as well as how to set up Time Machine and test your Time Machine backups. Apple got a lot of things right when they created Time Machine. It’s by far the easiest backup system to set up, monitor and use to restore a file. Having said that, it does have some short-comings including the fact that it doesn’t let you automatically switch between multiple hard drives and doesn’t let you encrypt your backup data. I describe how to manually switch between drives but Jay offers a way to automate the process. I have not tested his method myself but I have successfully set up encrypted Time Machine backups.

Jay provides excellent step-by-step setup instructions but here’s a quick overview.

You obviously need an external hard drive on which Time Machine will store the encrypted data. (Time Machine comes with Mac OS X 10.5 or newer.) Next, you use Disk Utility to create an appropriately named encrypted sparse bundle disk image which you put on the external hard drive. Save the password for your disk image and then move this saved password from your own Login keychain to the System keychain. The disk image then needs to be tweaked slightly so Time Machine will be able to figure out that it can store your data in it. This is accomplished by creating a custom preference (plist) file and putting this plist file into your Disk Image. Finally, select the external hard drive in Time Machine Preferences and Time Machine will magically store the backup data inside the disk image.