Use Multiple Backup Disks with a Single Mac
Time Machine lets you select one, and only one, destination disk at any given time. I would love to have the capability to automatically rotate among different destinations—say, two different hard drives and a network volume—or even to back up different files to different destinations, but such arrangements currently require using a more conventional third-party backup program.
However, you can manually switch between Time Machine destinations. And that's what you must do if you want to maintain copies of your Time Machine backups on two separate drives, in order to keep one of them off-site (see Off-Site Storage, earlier).
To use more than one disk with a single Mac, follow these steps:
Set up Time Machine ...
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