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I’ve been having trouble with Time Machine lately. For the past couple of months, it’s been deciding to do a complete backup of my system every week or so, purging all older backups (since it needed the room for the new full backup). This cost me several months worth of backups – I used to be backed up to last November. Now, I only have a couple weeks back. Fortunately, I rarely need to go more than a week or so back, but it sure is nice having those older backups.

But just as annoying – why was this happening? I couldn’t seem to figure it out. I’ve tried some of the great advice on the OS X and Time Machine site maintained by James Pond as well as Apple’s official support site and discussion forums, but to no avail. I tried resetting Time Machine,  restarting my system, running disk repairs – nothing worked. But by observation, I finally figured out what was happening, even though I don’t know why.

Part of my backup strategy is to have copies of my backups off-site. I have a pair of disks with 2 partitions, one a 1 TB partition that I copy my boot drive to, and the other a 2 TB partition with condensed disk images of the other drives as backups. I have 2 drives like this because I rotate then off-site approximately every week, so that one copy is off-site and another is on-site for backups. Usually the drive is disconnected on a Friday, and on Saturday I get back the other drive.

For some reason, starting a couple of months ago, Time Machine decided that when I ejected the two partitions on this backup drive, that my boot drive should be added to the Time Machine exclude list! Then, when I hooked up the other backup drive on Saturday, that drive’s boot partition was added to Time Machine as the only disk to back up (I only backup my boot drive with Time Machine; I use clones for all the other drives). Seeing a new drive every week, it decided it needed to whack the old backups because there was not enough space to backup the entire “new” drive. So for a couple of months this went on – I never noticed, as Time Machine was backing up every hour (and since it was backing up a cloned backup, it was going pretty quickly).

I finally noticed after Time Machine failed on a Friday evening, after I had disconnected the backup drive. I tried manually starting the backup, and was told there were no drives available to back up. So I checked the exclusion list, and my boot drive was in it! I removed that drive from the exclusion list, causing Time Machine to do yet another full backup – because it hadn’t been backing that drive up for a couple of months, it had been backing up the external backup drives.

I paid attention, and sure enough, the next Friday, after ejecting the backup drive, Time Machine failed – and again, the boot drive appeared in the exclusion list. I took it out of the list and was OK again (and this time, just a normal short backup, as I had been backing this drive up finally).

So now I know to check after ejecting the backup drive to check Time Machine. What triggered this odd behavior I don’t know. But at least now that I know about it, I can work around it.

The moral of the the story? Well, there are two of them, one general and one specific. If you are using Time Machine and have a bootable clone you frequently eject, you might want to check that your exclusion list hasn’t gotten messed up. And when you are making backups, don’t assume that the backup is good just because it doesn’t give you an error. Check on it once in a while, and even restore something from it every so often, so you know that the restore works.


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