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My external hard drive has failed. It will no longer spin up and therefore is unmountable. Thankfully I had added it to my OSX Time Machine backups. So, since disks are cheap, I can just go buy a new, probably larger external drive and restore, right? But how do I tell Time Machine that this is a replacement and to restore to the new drive?


PS - Yes I did remove the external drive from TM's exclude list and have verified that the backup is there on TM's own external drive.


PPS - Posted this same question on the Apple support forum and got a response that I just need to name the new disk the same as the old disk. This was not from an Apple employee. Any thoughts?

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2 Answers 2

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Naming the brand new drive the same as the old drive is the first step, but it doesn't get your files restored. After that you need to open the (empty) drive in Finder and then "Enter Time Machine" from the Time Machine icon in the Menu Bar. You can then go back to your last backup, select all the files and folders at the top level of your (old) drive, control-click on them and restore them. Prior to doing this I turned off Time Machine backups while this long restore was taking place.

I tried Migration Assistant but it didn't appear to have the ability to select a specific hard drive to restore.

I am in the middle of a restore using this method, and have yet to see whether Time Machine in Snow Leopard successfully identifies the new drive as the same as the old drive. When I tried this method last year in Leopard, Time Machine decided that the identically-named drive was a new drive with the same name -- I ended up with two backups named "Ext HD" and Ext HD 1", one of which was the old, discarded drive, and the two together doubling the amount of space they used on my backup drive. You can control-click on an item in Time Machine and have it delete all backups of an item if you end up in this situation, but then you'll lose all your old archival backup data for the drive.

Also, I suspect this method misses hidden files at the root of the drive -- if you've changed the drive icon it isn't restored, for example. If that's a problem I believe you can navigate up one level to the machine in the Time Machine window, select just the external hard drive backup, and restore everything, but it will then be in a folder named after your external hard drive. You'll have to manually move both the visible and hidden items once the restore is done.

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My restore finished, and Time Machine does not recognize the restored drive as identical to the original -- it attempts to backup the entire contents again from scratch. –  Ert Feb 13 '11 at 1:26

Naming the new volume the same as the old one makes it very easy to go back one step in time and restore the contents of that volume's root directory.

What the restore doesn't seem to do is copy over the extended attributes (com.apple.metadata:_kTimeMachineNewestSnapshot and com.apple.metadata:_kTimeMachineOldestSnapshot)

that Time Machine uses to decide whether it needs to re-backup a file. So when you turn Time Machine back on with the new volume in place, it backs up another copy of every single file instead of creating just another set of hard links to the copy it already has. If you have a backup drive that is big enough, then this is ok (not ideal, but ok). If your backup drive doesn't have the additional room, then you've potentially got a problem.

I haven't tried restoring the files using Unix commands (instead of Time Machine's "Restore" button) to see if one can keep the extended attributes in place. I believe that they are kept in the Time Machine backup so in theory one should be able to retain them and make Time Machine understand that it still has an unchanged copy of the files.

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For a command line restore with Lion or greater, try the restore verb of tmutil. –  Graham Perrin Jul 24 '12 at 8:54

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