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I have backed up my Mac to an external disk using TimeMachine. When I plug in a different external disk the Mac still expects to backup to the first disk. Is it possible to backup to two external disks, one at a time?

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up vote 7 down vote accepted

You just have to select both disks as backup disks from Time Machine Pref pane, one at a time.


Attach your first drive, open the Time Machine system preference, click the Select Disk button, and choose the attached drive as the backup destination. Initiate a backup and wait until it’s done.

Now attach the other drive, open the Time Machine system preference yet again, click Select Disk, and choose the new drive as the backup destination. Again, wait for the backup to complete. You now have two identical backups.

Taken from Macworld Article

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Actually, this have changed with Mountain Lion, and now the answer is very simple:

You can backup both at same time! \o/

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Assuming you're trying to have two or more rotating backup sets, there's another method which may be more convenient (taken from this macosxhints forum post):

  1. First set up a RAID 1 between the two disks, using Disk Utility.
  2. Set the RAID set as your Time Machine target drive, and let it do a full backup.
  3. Unmount the RAID set, unplug one of the drives. Re-mount the other drive. The RAID set will now be running in "degraded" state, meaning it only has one drive.
  4. Take your other drive and put it somewhere safe.
  5. After a week, or whatever period, bring the off-site drive back, connect it, and let the RAID re-build.
  6. Repeat steps 3-6.

I've just recently started this plan, and rebuilding a 2TB RAID takes a really, really long time: roughly 40 hours.

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