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Is it possible to merge to Time Machine Backups?

When I look inside the sparse-bundle which Time Machine generates then I see that the folder-structure ordered by dates. So theoretically I could copy the files from my old TM sparse-bundle into the new one. Is this possible?

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    I think a "why" you want to do this is in order.
    – Jason Salaz
    Commented Jan 6, 2011 at 20:58
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    Not officially supported. If you want to try, I would clone (backup) one of your Time Machine disks, then experiment.
    – user588
    Commented Jan 6, 2011 at 21:09
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    Time Machine is using hard links, I'd be VERY careful in trying to mess with its internal storage.
    – Thilo
    Commented Jan 7, 2011 at 2:38
  • Conceptually it's absolutely possible to think of a program that would re-do the work to set up the Time Machine compression and merge both. In practice, such a tool isn't publicly available.
    – bmike ♦
    Commented Jul 17, 2011 at 16:19

3 Answers 3

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I tried: it works.

Here is how:

I have a time machine volume and a backup of it. I update this backup every two months, using Super Duper.

Recently, several successive mistakes made my time machine volume loose its first two years of backups. I then had a primary time machine volume with all backups but first two years and a backup time machine volume with all backups but latest two months.

I decided to experiment. Starting from scratch on a new hard drive, I copied the backup time machine volume to the new hard drive with Super Duper (backup all files using erase and save option). Then I copied the primary time machine volume on top of that, again with Super Duper (backup all files, using copy files that are different).

I ended with a new working time machine volume including all backups, from day one up to today.

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  • I am curious if Super Duper was smart enough to manage the hard links. I am guessing you ended up with more space used than you needed. Would you mind grabbing a copy of BackupLoupe and having it chew on your "merged" store? Even knowing it half way worked - preserving the links from the old set and preserving the different links in the new set would be an improvement over the wort case.
    – bmike ♦
    Commented Jul 17, 2011 at 16:18
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    Super Duper is known for handling correctly Time Machine volumes, including hard links. This is why I use it for making backups of time machine (which helped me a couple of times). About space, my new time machine volume takes up 400G, while my old ones took about 300G and 350G. I didn't take the time to scan my new time machine with BackupLoupe but a quick ls -i on all versions of GraphicConverter.app (Which I got bundled with Panther and never updated) reveals a difference in inode at a date related to my first mistakes, not to first or last dates of my two time machine backups.
    – mouviciel
    Commented Jul 17, 2011 at 21:29
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    Wow - I'm certain that SuperDuper is able to preserve hard links that exist on one volume - but matching them to different hard links on another volume seems almost too good to believe to be true. Thanks so much for checking!
    – bmike ♦
    Commented Jul 17, 2011 at 22:08
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Simply: Don't!

Time machine uses hardlinks, that you will not be able to recreate by simply copying stuff around. What this means in plain english is you might end up with each copy of the file taking up space - rapidly filling the drive.

Time Machine uses great care to only store each file once - and only takes up more space when the file changes. It assumes everything already stored is correct and won't go back and "fix" any errors you make. It's possible to make a tool to merge the backups, but Snow Leopard makes it easy to browse both backups. Just connect all the drives and option click the Time Machine icon in the menu bar to browse the other backup drives and see that history.

Many people have their time machine data on several drives - you can safely put them on the shelf and move to a new drive rather than go back and edit the past. It's easy to move the whole package to a new drive if needed - just not splice data in to an existing backup folder.

A great place for information is this time machine and internals site and also the Time Machine discussions forum at Apple (I'm assuming we're talking about Snow Leopard here).

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Time Machine does not (or should not) work when you tamper with it's files in any way. Even a cloning of the TM backup and restoring it can (and probably will) make Time Machine not work with the backups anymore.

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  • I've not had any issues with following the steps at support.apple.com/kb/ht1427 for moving the files to a new home. It's usually some other problem when it stops working (which of course could have been caused during the move - but certainly not probably will cause issues)
    – bmike ♦
    Commented Jul 17, 2011 at 17:27
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    @bmike, that support page no longer says anything about moving the backup set.
    – WGroleau
    Commented Nov 13, 2020 at 19:50

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