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I've recently had a MacBook Pro need to have its logic board replaced, this resulted in certain identifiers of the machine changing (MAC address, etc) so Time Machine no longer regards it as the same machine, and will no longer perform backups.

I have restored from the backup without any problem, and was hoping this would prompt it to realise there was a single point at which both the computer and the backups were in synch, and would hence continue to perform incremental backups from that point on, but it stubbornly refuses to allow backups.

If I have to lose the old backups and start afresh, I'll be able to cope somehow, but I would prefer to keep the history if possible.


locked by HopelessN00b Dec 5 '14 at 3:44

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up vote 3 down vote accepted

There's full instructions on how to do it at this answer from


As Alnitak pointed out, has instructions for doing this. Here's another related hint, which comes with a very large "DISCLAIMER: Do this at your own risk!" that should probably adorn both of them.

(I would have simply added the hyperlinks to Alnitak's answer, but we're all starting over from a meager 101 reputation here, so no editing of other people's answers yet…)


Based on the data at the provided links from Alnitak and John's earlier answers, here is what I did:

  1. Plugged in the (USB) Time Machine HD
  2. Turned Time Machine off
  3. Found the current ethernet MAC address
  4. Disabled the ACLs on the TM
  5. Moved the .[old mac address] file in the root of the TM to be .[new mac address]
  6. Updated the extended attribute for the Backups.Backupdb/[machine name] to match the new mac address
  7. Reenabled the ACLs
  8. Ejected and unplugged the TM HD
  9. Turned Time Machine back on
  10. Plugged the HD back in again

Steps 4 through to 7 were done as follows:

$ sudo fsaclctl -p /Volumes/Time\ thingy/ -d
$ mv .[old mac address] .[new mac address]
$ sudo xattr -w [new mac address (with colon separators)] Backups.backupdb/Thalia/
$ sudo fsaclctl -p /Volumes/Time\ thingy/ -e

It ran the backup OK (looking at the inodes it recreated the entire directory structure, but linked the files to the previously backed up versions where it could, so not flawless, but good enough), and I can now "Enter Time Machine" and see the expected history.

As a bit of a side note, before resetting the xattr I ran xattr -l (list) on the Thalia directory (looking for any other attributes that might be around to cause problems), and noticed that the pre-existing mac address was displayed in hex editor style (the address column (0000 on one line, 0010 on the next), the hex data column (each byte separated by a space), and the data column) and the data was actually terminated by a zero byte. After resetting it, it is just displayed simply as the mac address string (so presumably not terminated by a zero byte) - I'm hoping this won't cause problems.


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