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I've just replaced my existing MacBook Pro with a new MacBook Air. I've got 18 months of Time Machine backups from the old computer. I'd like to configure my new computer to use the old computer's Time Machine backups, so the new computer picks up where the old one left off. Is this possible? If so, how?

I didn't restore the new computer from a Time Machine backup because my old computer used more disk space than my new computer. There was a lot of cruft as well, so I decided to start from scratch on the new one. I'm slowly pulling off individual files and settings from a clone of the old computer's hard drive.

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So what do you mean by "use the old computer's Time Machine backups" if you don't mean restoring from it? –  Daniel Lawson Dec 3 '11 at 4:15
    
The backups from my old computer use 1.1 TBs of a 1.5 TB drive. If I start backing up my new computer from scratch, the new backups will be limited to the remaining 0.4 TBs. If I tie the new computer to the old computer's backups, I get access to the old computer's files (which is what I want, since conceptually the two machines are the same) and Time Machine will delete old backups to free up space. –  splattered bits Dec 6 '11 at 19:55
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3 Answers

up vote 4 down vote accepted

I found the following solution here:

sudo tmutil associatedisk -a "/Volumes/Macintosh HD" "/Volumes/Time Machine Disk/Backups.backupdb/John Doe's MacBook/Latest/Macintosh HD"
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  1. Make sure the name of the new computer's hard drive is the same as the old computer's hard drive. Click once on the hard drive and hit enter to rename.

  2. Make sure the new computer's name is the same as the old computer's name. Open System Preferences, then Sharing. Enter the name in the Computer Name field. (This step may not be necessary, but it's what I did.)

  3. Plug-in the existing Time Machine drive. Open the Time Machine preferences and turn Time Machine on. Click Select Disk… and choose the Time Machine drive. After a few seconds, the backup will start. It will probably take awhile because OS X recognizes that you're backing up an entirely new disk, so backs up the entire disk. It might have to delete a few of your oldest backups, depending on how much space is left on the Time Machine drive.

After the first backup, you can safely change the name of the new computer and its hard drive.

Update: Using this technique, you won't be able to browse any backups from your previous computer with the Time Machine interface (although they are removed when Time Machine needs to free up space for new backups). The old backups are still visible and accessible from the Finder.

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I don't get what are you trying to accomplish with all this renaming business. As far as I know Time Machine recognizes backup sources by UUID and not name of the hard drive. Did I missed something? –  iskra Dec 6 '11 at 18:49
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It does use UUID, but also watches the name of the disk. If OS X detects the UUID of a disk changes, it falls back on the disc's name to pair a disk with a backu. The first backup after the UUID changes, it will do a full backup of the new disk (because it knows the disk is different), but as part of the history of the old disk. –  splattered bits Dec 6 '11 at 19:46
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If you want to back up to the same Time Machine disk that your old computer did, you can do that by plugging the external disk in and selecting it for Time Machine. If you don't restore from that backup, however, Time Machine will house both backups on the same volume, but you won't be able to pull files off of the drive from the other computer through the Time Machine interface. You can get to the other computer's backups by accessing the drive through /Volumes in the Finder.

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This isn't what I want. I don't want to just use the same disk; I want to use the same backups. –  splattered bits Dec 6 '11 at 8:17
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