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I'm going to be migrating to a new machine in the new few weeks/months, and want to avoid something that happened during my last migration.

I used Migration Assistant and a Time Machine backup to load everything on to my then new(ish) MacBook Pro. Everything copied correctly, however, when I tried to set up Time Machine on the new MacBook Pro using the original Time Machine disk, I was unable to. I'm not sure why, and as a result, I had to buy a new external HD for a new Time Machine backup. This was a somewhat of a waste because the original Time Machine backup disk was less than half full. Also, I was no longer able to access backups from before the migration, since the new MacBook Pro would not recognize the original Time Machine disk.

Now, I'll be doing the same thing soon, and my current Time Machine disk is barely a third (1/3) full, and I'd rather not purchase a new HD, and also rather not have to lose a few years worth of backups.

Is it possible to re-establish Time Machine for a newer machine using the same disk as for the older machine (and the source of the migration image)?

  • Why buy a new disk? After the migration you have your old mac, the new mac, and the external time machine. This is safe enough to erase the external disk and use it as a new backup volume. – Thomas Jan 24 '18 at 22:27
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When you get a new Mac, you can use Migration Assistant, along with your Time Machine external drive, to restore your a backup to your new computer. However, Time Machine will not backup your new computer to that external drive because it recognizes that the identifier of the internal drive on your new computer doesn't match the identifier of the internal drive on your old computer. (It's trying to prevent another disk with the same name from being backed up and causing problems with existing Time Machine backups.)

You have two options to get around this:

  • You can format the Time Machine drive after you restore, and then set it as the Time Machine destination as normal. Of course, this means all of your backups from the past are gone.

  • You can associate your new internal drive with Time Machine, thereby allowing it to continue with backups.

There is a chance that something goes wrong in the association process and you lose all of your previous backups. But it should work, and certainly formatting the disk will result in no chance of you having access to your previous backups.

To associate your new computer's drive with your Time Machine drive, do the following:

  1. Ensure that you have restored your backup onto your new computer and everything seems present and accounted for. Again, you should come out of this with all of your backups, but you want to be sure everything's already on the internal drive, just in case.

  2. Find out where your backups are. Go to the Time Machine external drive in Finder and double-click on Backups.backupdb. Double-click on the folder with the name of your computer, which should be the only folder present. Scroll to the bottom until you find Latest and double-click on that. At this point you should see a folder with your hard drive's name, and double-click on it.

  3. In the View menu, select Show Path Bar. (If the only option you see about the Path Bar says Hide Path Bar, ignore the last sentence) At the bottom of the Finder window you should see something like this:

    enter image description here

    At the far left is your Time Machine disk's name, followed by the .backupdb folder name, followed by your computer's name, followed by the date-time of the last backup, followed by your internal hard drive's name.

    Keep the Finder window open.

  4. Note that you'll need to replace several bits in the following command with your computer's information. It's important that you do not add or remove spaces or change anything other than the bits indicated.

    sudo tmutil associatedisk -a "/Volumes/New Internal Hard Drive Name" "/Volumes/Time Machine Disk Name/Backups.backupdb/Computer Name/Latest/Old Internal Hard Drive Name"
    

    In the above command, this is what needs to be adjusted. Here's the image of mine again for you to follow along with:

    enter image description here

    • New Internal Hard Drive Name: This is the name of your new computer's internal hard drive. If you haven't changed it, it will be Macintosh HD by default. (This is not in the little image above, but if you haven't moved it, it's the name on the drive icon in the upper-right corner of your desktop on your new computer.)
    • Time Machine Disk Name: This is the name of the external drive that you use for Time Machine. It's whatever shows up in Finder when you go to view your Time Machine drive, including spaces. (This is Baracuda in the image above.)
    • Computer Name: This is the name of your old computer. If not changed from the default, it will be something like Joe's MacBook Pro. (This is KevG5 in the image above.)
    • Old Internal Hard Drive Name: This is the name of your old internal hard drive. (This is Ted in the image above.)

    Adjusted for my computer, this is what it would look like:

    sudo tmutil associatedisk -a "/Volumes/Bill" "/Volumes/Baracuda/Backups.backupdb/KevG5/Latest/Ted"
    

    Again, you must adjust it with your own computer's information. If you have questions, please use the comment feature below this answer.

  5. Now, open Terminal (in the Applications > Utilities folder), copy and paste your adjusted command, and press Return/Enter. You will be asked for your computer login password. When you type it, the characters will not appear - that's to be expected. After you type your password, press Return/Enter again.

After all of this is done, you should be able to use your existing Time Machine drive with your new computer and pick up with backups as if nothing else changed.

  • Thanks so much for this! Very thorough and helpful! Is it just that Apple wants us to buy more external drives, or is this some sort of shortcoming of OS X? – iwantmyphd Oct 4 '16 at 1:06
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    Neither. As noted in the post, it's intended to prevent an accidental backup of another disk with the same name. If you take your external Time Machine drive and hook it up to somebody else's computer, and you both have your internal drives named "Macintosh HD", without this protection, running a backup there would create a whole new snapshot. Depending on the size of the drive to be backed up and the Time Machine drive, it might erase all existing backups to create that new backup, and suddenly you've lost all your backup history. As for why this feature isn't exposed in the user ... – tubedogg Oct 4 '16 at 1:10
  • ... interface, I think Apple probably feels most people would rather start with a clean backup history with a new computer. – tubedogg Oct 4 '16 at 1:11
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    how would you do it for a distant time machine disk ? I get "A local volume mount point and a snapshot volume path are required." as a response. – Kamel Aug 22 '17 at 8:07
  • @tubedogg I added the special use case "Continue using TimeMachine on old Mac after full restore to new Mac". I think is worth to be included in your already excellent answer, as I found it nowhere online and my investigation took me a whole day. – porg Mar 3 at 0:44
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Apparently with macOS Sierra your new Mac can inherit your old Time Machine disk and likely with High Sierra, too. I recently moved the main drive from a failing 2011 mini to a new-to-me 2012 mini. I was prepared to erase the Time Machine disk and start over, as suggested by the first commenter, but I plugged it in and out of curiosity was looking at its contents in Finder. A dialog message popped up asking if I wanted this computer to inherit the Time Machine backup from my previous computer. It cautioned that if I wanted to do this I could no longer use the backup on the previous computer. Fine with me. My previous backup is now associated with the new computer with no problems.

Apple support doc outlining when and where you can inherit a Time Machine disk: https://support.apple.com/kb/PH25698?locale=en_US

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Hi I had to do a couple of things to do before being able to see my previous archives on my new Mac's Time Machine.

For history here is what I did :

Run the sudo tmutil associatedisk -a on terminal

Had the following error:

Failed to gather snapshots for xxx

It was a problem of terminal permissions !

So I did this:

I went into System Preferences > Security & Privacy > Privacy and added Terminal to the Full Disk Access panel. Worked for me. Hope it helps!

(thanks https://forums.macrumors.com/threads/unable-to-inherit-previous-time-machine.2144061/)

Then the tmutil associatedisk -a worked... but I couldn't see my previous backups on time machine

And this made it work

First I had to inheritbackup of that sparesbundle: sudo tmutil inheritbackup "/Volumes/Backups/OldComputerName.sparsebundle" After that, I could use the associatedisk option like tubedogg mentions

specifically for my computer:

tmutil inheritbackup /Volumes/WD\ 2TB\ BACKUP\ 2/Backups.backupdb/MacBook\ Air\ xxx

then

sudo tmutil associatedisk -a /Volumes/Macintosh\ HD /Volumes/WD\ 2TB\ BACKUP\ 2/Backups.backupdb/MacBook\ Air\ xxx/Latest/Macintosh\ HD/

And BAM! I could browse my backups with timemachine

  • +1 for the terminal permissions ! Other than that I've followed @tubedogg procedure :) – AnthoPak Dec 16 '18 at 21:11
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I also had this problem. However, my Time Machine backup was on a networked drive, and it was part of a sparesbundle. I found this site https://nairteashop.org/keeping-backup-history-when-switching-to-a-new-mac/ which explains how to do that.

First I had to inheritbackup of that sparesbundle: sudo tmutil inheritbackup "/Volumes/Backups/OldComputerName.sparsebundle"

After that, I could use the associatedisk option like tubedogg mentions

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