Time Machine has always been able to work with multiple backup devices, but until the release of OS X Mountain Lion, it was never an easy task. It required quite a bit of user intervention to get the process working correctly.
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That's all changed. With Mountain Lion, you can easily add more than one backup drive for Time Machine to use, ensuring a form of redundancy in your backup plans.
Having more than one Time Machine backup drive may seem like a waste of a good drive, but if you have important data on your Mac, then maintaining a backup of your backup isn't an extreme measure.
In this guide, we examine how to use multiple drives with Time Machine, how Time Machine will make use of them, and how to restore information when you use multiple backup drives.
Time Machine With Multiple Drives - How to Use Two or More Backup Drives