Time Machine has been around for a long while now, it was first introduced in the Leopard (10.5) release. While there has been some changes under the hood from a user perspective it works now as it every has.
Along the way though there was one featured introduced that not many people know of: the ability to backup to more than one disk.
Why backup to more than one drive?
There are probably many circumstances but I first discovered this ability when I was questioning the integrity of our Drobo drive that I had been using with Time Machine for a couple of years. I wanted to be able to keep that going but swap it out for another external disk, just to make sure I had a good copy of the entire machine (in this case a Mac Pro running OSX server).
After a bit of reading I came across some information on the Apple Support Forums followed the instructions and was done in just a few minutes, well actually the backup took something like 12 hours – there’s a lot of data there!
For others though you could implement this if you wanted to leave one backup disk in the office an then take another home – an off site backup is always a good idea.
How to set up multiple disks
It’s actually really easy to do. The trick is to pay attention to the dialogue box that you’re given when choosing the disk in the Time Machine preferences. Assuming that you have it up and running with one disk, you go back into the preferences and again hit Select Disk.
Check out the screen shot below, here you choose Use Both. And that.. is it!
What happens now?
So OS X just rotates between the two disks. Now it will use the new disk and create a full backup image there. Next time Time Machine runs it will see what needs to be added to the first disk and just keeps swapping between the two when they are both connected.
If just one drive is there then it will use that one as soon as you add back the second drive it begins to rotate between the two again. All the while working out what each backup disk is missing.
When Time Machine is running head back into the preferences and you can see which disk is active at any given time, see the example in the screen shot below.