One of the great things about Time Machine is that it can store backups from multiple computers all on one hard drive. To do this, it needs a way to individuate each backup. One might notice that the backup name is the same as the computer name, aka “MyComputerName.sparsebundle”. However, it’s very easy for a user to change their computer’s name, and if two machines are on the same network with the same name the newest one is automatically reassigned a new computer name. For example, if I have a machine named “Rockstar”, and another machine joins my network it will automatically be renamed “Rockstar (2)” to avoid confusion.
The truth is, while Time Machine backups display the computer name as the prepend to .sparseimage, that is not actually how Time Machine differentiates computers; it uses the ethernet ID (aka MAC Address) of the machine that it’s backing up to tell the machines apart. This is very smart, but the catch is that if one’s computer has the logic board replaced the ethernet ID is different and Time Machine is unable to re-link the computer to its previous backups. While it’s easy enough to reconnect to the Time Machine drive and create a new backup, for someone like me who has over 150GBs on her machine, this can be an unnecessary time-suck. It _is_ important to note here for clarity’s sake that if you had been backing up your data using Time Machine and you lost your data during repair it is still certainly possible to restore from that backup, it just won’t continue to make backups on the same time-line if the ethernet ID is different.
Naturally, there is a solution! “Mac OSX Hints”:http://www.macosxhints.com/article.php?story=20080128003716101 has a great article on how to solve this issue. I’ve included the instructions below, just be sure not to dive into it if you are not comfortable using Terminal. This is also a “your mileage may vary” situation, as I’ve heard conflicting responses to its success, though anytime Terminal is involved there’s always a good margin for user-error.
Directly from “Mac OS X Hints”:http://www.macosxhints.com/article.php?story=20080128003716101 , with the addition of some proper edits from the comments:
bq. I will assume that your backup disk is named Time Machine and your computer is named MyMac. In addition, my “old” MAC address for demonstration purposes is 00:f9:e8:d7:c6:b5 and the new one is 00:1a:2b:3c:4f:56. Change occurrences of those values below wherever they occur to match your system.
bq. The last bit of information needed is the old MAC address. Open Terminal and change directory to the root of the Time Machine backup database, then get the old MAC address:
cd /Volumes/Time Machine/Backups.backupdb
xattr -p com.apple.backupd.BackupMachineAddress MyMac
bq. Write this number down (or copy it) as well. Change directory to the root of the Time Machine disk to verify that the MAC is correct:
$ cd ..
$ ls -al
bq. Near the top of the listing, you should see a file that begins with a period that matches the old MAC address, without the colons between digit pairs –.00f9e8d7c6b56 using my example MAC address.
bq. Now to get to work. Insert your system’s data for the following commands to disable ACLs temporarily so changes can be made, rename the hidden system ID file, reset the extended attribute on the backup store, and re-enable ACLs:
bq. $ sudo fsaclctl -p /Volumes/Time Machine -d
$ sudo mv .00f9e8d7c6b5 .001a2b3c4f56
$ sudo xattr -w com.apple.backupd.BackupMachineAddress 00:1a:2b:3c:4f:56 Backups.backupdb/MyMac
$ sudo fsaclctl -p /Volumes/Time Machine -e
For those of us using a Time Capsule for backups, there is a solution for us as well! Again, this is taken directly from “Mac OS X Hints”:http://www.macosxhints.com/comment.php?mode=display&format=threaded&order=ASC&pid=97623 :
bq. In the proceeding code, let’s assume that that “Time Capsule” is the internal disk of your Time Capsule, and “Backup of MyMac” is the name of the sparsebundle disk image volume that mounts when you normally back up to your Time Capsule. First off, double-click your old (correct) sparsebundle disk (MyMac_00f9e8d7c6b56.sparsebundle) image and mount that volume on your Desktop. Also, to reiterate, old MAC = 00:f9:e8:d7:c6:b5, new MAC = 00:1a:2b:3c:4f:56, as they are above.
bq. $ sudo fsaclctl -p /Volumes/Backup of MyMac -d
$ sudo xattr -w com.apple.backupd.BackupMachineAddress 00:1a:2b:3c:4f:56 /Volumes/Backup of MyMac/Backups.backupdb/MyMac
$ sudo fsaclctl -p /Volumes/Backup of MyMac -e
$ cd /Volumes/Time Capsule
$ sudo mv .00f9e8d7c6b56 .001a2b3c4f56
$ sudo mv MyMac_00f9e8d7c6b56.sparsebundle MyMac_ .001a2b3c4f56.sparsebundle
I hope this helps some of you out there! Keep backing up!!
