A few years ago you wrote about creating multiple Time Machine backups for a single Mac. You explained that you needed to manually choose the different volumes you’d use for backup. I too want multiple backups of my Mac’s data and was hoping the process was easier now.
If your Mac is running Mountain Lion, it is easier. It goes like this.
Designate another volume as a second backup destination—a hard drive attached to your Mac or a network volume, for example. Launch System Preferences and then select Time Machine. Click the Select Disk button and you should see not only the volume you’re currently using for your backup but also any volumes available to you (that second destination you just added being one of them).
Select that volume and click Use Disk and a sheet will appear asking if you’d like to replace your original backup destination or use both volumes. Click on Use Both. Time Machine will then set about creating a backup archive on the second volume and then back up your Mac to it.
Adding a second backup volume to Time Machine
From that point on, it will alternate its backups between the two volumes—back up to Volume A and then, an hour later, back up to Volume B. For example, if you disconnect one of the volumes, Time Machine will continue backing up to the one it can access. When you bring the previously disconnected volume back on line, Time Machine will back up to it so that the two volumes hold the same approximate data.
I say approximate because if you’ve created new documents since backing up to the just-brought-online volume, this new backup will be more current than the other one. And that’s the minor inconvenience of this scheme—one backup set is likely to be a bit ahead of the other, which means paying attention to which set you restore from.