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Monday, October 31, 2011

SuperDuper! Pro Tip

As much as I love my new MacBook Air, the transition to the new machine hasn't been completely seamless. Like any transition to a new computer, there are bound to be hiccups. One of these hiccups was around my backups. Since I take my backups very seriously, this was something that really bothered me until I was able to figure out what was going wrong.

After migrating all of my data to the new machine I made sure to run SuperDuper! (the best disk cloning software in the universe). It ran perfectly for quite some time but after a few weeks it started to fail by running out of disk space on the backup drive.

When I first partitioned my backup drive I made the clone parition the same size as the boot drive: 120GB. But the MacBook Air has a larger drive in it, double the size at 250GB. I've been careful to make sure that I don't use more than 120GB on the new machine, so surely the problem wasn't that I was using more space than I should be. Or at least that's what I thought.

It turns out that Lion comes with some enhancements to Time Machine. Under Lion, if your backup drive is not connected, Time Machine will continue to run. The backups are stored in /.MobileBackups -- a hidden folder at the root of the boot drive. Since this is effectively temporary storage, Lion doesn't report any of the space used by this directory in the Unix disk free or df command or in the Finder. Essentially, Lion hides the fact that it's using this space from you at all. It does this because it will automatically remove old backups to make room for new files written to disk if need be.

Since SuperDuper! didn't know this it would attempt to copy this directory to the backup drive. Ordinarily this wouldn't be a problem since the clone drive is usually the same size as the drive being cloned. This wasn't the case for me however. After I figured this out I created a new backup script and excluded the /.MobileBackups directory. After applying the changes I haven't had any problems.

So, if SuperDuper! is complaining about running out of disk space when running on Lion, you might want to either:

  1. ensure that the size of the drive you're cloning from is the same size as the drive you're cloning to. or;
  2. exclude /.MobileBackups from your SuperDuper! backup script.