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I have been backing up a MacBook Pro to an external hard disk with Time Machine. I got a new laptop, used the firewire connector to pull the universe across to it, and started it up. It does not want to just pick up where I left off with the backups; it wants to start a new backup sequence and thus I need a ton of additional disk space.

Does anyone know a way to force it to just incrementally back up to the existing backup set?

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Good you found a solution for your situation. Still, for the archives: in general, I would not manually try to trick OS X in using the backup of some Mac as a base for backing up another Mac. In the end, for me, being able to fully restore in case of some disaster is more important than keeping some history or saving some hard disk space. Also note that holding down Option while clicking the Time Machine icon allows for browsing files on any Time Machine disk. –  Arjan Dec 19 '09 at 15:09
    
superuser.com/a/538477/7821 –  porneL Jan 19 '13 at 1:40
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4 Answers 4

up vote 8 down vote accepted

It turns out that under Snow Leopard, Apple has allowed for this. When you turn on TM for the first time, it will ask you:

Would you like to reuse the backup [..] with this computer?
The backup was created on a different computer. If you reuse this backup it can no longer be used by the original computer.

I was mislead by the wording, but it turns out to mean just what's called for here: continuing the backup sequence on the new machine as if it were the old machine.

Snow Leopard reuse backup

Image from blog.gerrior.com, in an article about logic board replacement (probably using the very same hard drive!)

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I would imagine this only works when you've used that same backup to restore things onto the new system, like you described. (Or when the hard drive is still the same, like after logic board replacements.) Still then, not all Macs are equal, so I surely hope OS X ensured all (system) files that are currently on your Mac are also on the backup. –  Arjan Dec 19 '09 at 14:41
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This came up on Mac OS X Hints under the title 10.5: Repair Time Machine after logic board changes. Never tried it and I'm not sure if it would work on 10.6. Use with care.

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This is the right thing for Leopard but now for SnowLeopard, see my subsequent self-answer. –  bmargulies Dec 3 '09 at 13:26
    
Ah, good catch! –  fideli Dec 3 '09 at 15:15
    
I have tried it on 10.6, and it works. –  Kevin Vermeer Jun 29 '11 at 22:47
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Would comment, but I lack the rep...

bmargulies is correct about this function in Snow Leopard. It actually works quite well- when the logic board is changed, as described the MAC address of the network interface also changes, "breaking" the association w/ TM. In the case where the TM backup prefs are managed by Snow Leopard Server, you'll have to update the MAC address of the relevant computer record in Workgroup Manager before the machine with the new logic board will "remember" that its associated with Time Machine and popup the dialog mentioned by bmargulies.

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"Reconnecting" to your backups article has very good information.

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