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I have a Time Machine backup, and have bought an identical external drive, with the intention of running TM on both drives.

I know that I can just plug the new drive in and configure it as a TM target, but I have a significant amount of history in the backup that is no longer on the source system (due to disk space constraints). I'd like to start the redundant backup as a mirror of the original, so that the older weekly data is available from either TM. Ultimately, one of the disks will be at the office, the other at home. Anything else isn't really a satisfactory DR solution.

I've found any number of discussions about cloning TMs and moving TM from one disk to another, but not an answer to this question. Perhaps it's perfectly straightforward and I'm over-thinking it. But I don't want to have these disks chugging for hours copying data that isn't going to produce a second functional TM.

Thanks in advance.

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The short answer to my question is "Yes, you can". The tricks involved are:

  1. Make sure the second disk is formatted identically to the first (ie if the first one uses Apple Partition Map or GUID Partition Table, the replica must be the same. Disk Utility reports inscrutable non-obvious errors otherwise.)
  2. Carbon Copy Cloner probably will work, but after it churned for 8 hours, only 8.5GB had been copied. Using Disk Utility, all 300GB was copied and verified in the same time. That's not to diss CCC, it's very good at what it does, but for some reason it was very slow for my case. Others say DU didn't work for them, so YMMV.
  3. At the finish, I had two identical disks. I renamed them both, and each was successfully recognised as a TM drive when plugged in, with its new name. I had to reset their Spotlight non-indexing status, but otherwise they worked exactly as they should.
  4. I ran a "Backup Now" on each (serially), and each reported and processed the same volume of data to be backed up. Both took exactly the same time to calculate the work required and perform it.
  5. The Time Machine dock item worked exactly as it should for both disks, showing the correct data from the just completed "Backup Now" job.
  6. The Preference Panel reports the timestamp of the last one done, irrespective of which drive was used.

So, I now have a workable swap-out setup for my backups, where one of the disks stays offsite with me and the other plugged into the server. Thanks everyone for taking an interest.

  • I'd love to know if I'm wrong, but I'm 99% certain that the two disks will drift apart from each other over time, so you may have to check both if you want to restore a file and can't find the version you want on the drive you have handy. The differences between the two may be unimportant to you, but unless there's some magic I'm unaware of, I'm pretty sure they'll diverge, especially if there is a noticeable lag between swapping them back and forth. The upsides of your approach might vastly outweigh this downside, but nevertheless... – iconoclast May 14 at 18:01
  • Also, is there an advantage to giving them the same name? It seems you can do almost the same thing with different names: maketecheasier.com/time-machine-multiple-drives-backup-mac – iconoclast May 14 at 18:12
  • It was ten years ago that this question was posed. Support for connecting multiple TM disks has been part of the Preferences panel for a long time now. It just wasn’t in 2009. – RET May 15 at 21:33
  • ahhh... yes, thanks for putting that into temporal perspective. – iconoclast May 16 at 0:32
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If you clone your Time Machine backup (using something like Carbon Copy Cloner or a restore using Disk Utility) both will be functional Time Machine backups and contain the same history as of that moment.

You can use both disks - if they have the same name and they've been cloned the Mac will recognize it as the Time Machine backup drive to do backups to. Without having done it myself I can't say for certain whether the first backup will take longer as it will rescan what has been backed up and what hasn't as the computer's records of it's last backup don't match the hard drive, but it will back up the files that have changed that day - you will just loose the granularity of what time it was changed. (eg. the differences between noon and 1PM at home, the differences between say 6PM and 7PM at work as the home/work hard drive would not be connected).

  • 1
    Thanks, I appreciate your reply. I've seen suggestions like this elsewhere, but there's generally a huge caveat that the two HDs should never be plugged in simultaneously. 10.5.6 does support multiple backup disks. I can do as you've suggested, rename one of the disks and see if TM is happy to work with it. But I'm really hoping someone has been down this road before and can make a definitive call on it. – RET Jul 17 '09 at 16:33

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