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Imagine me showing up at my friend's doorstep with a laptop in several charred pieces, an intact external drive with (phew) Time Machine backups, and a deadline in a few hours. "Quick," I say, "give me your laptop!" I plug the external drive in and have it acting like my own laptop (rest its soul) in a matter of minutes.

Is that possible? Without wiping my friend's laptop, of course. And ideally without a second external drive.

As you know, if you don't do restores you're not really doing backups. So I'd like to try this out to make sure it would play out as in the scene above.

The question is, how, exactly?

PS: Here's a related but more pie-in-sky question:

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Edited my answer for your most recent edit. – Daniel Beck Oct 30 '10 at 11:32
Sorry for the edits to the question. I know that can be annoying but I think the question is much simpler and clearer this way. And thanks so much for all the help with this! – dreeves Oct 30 '10 at 11:38
It's not clearer, it's a whole different question. Twice. Reflected by the now unsuitable title. – Daniel Beck Oct 30 '10 at 11:50
This is the one I was trying to ask all along but I obviously did a bad job. Good point about the title; I'll change it. Originally the option of booting from an external drive hadn't occurred to me, which is why I was asking about restoring to a friend's laptop. But the point all along was about how to use a borrowed computer temporarily if yours dies. So I think now the question is strictly more general. – dreeves Oct 30 '10 at 20:20
Makes sense. Could we at least provide an answer or is something still unclear? I kind of lost track. – Daniel Beck Oct 30 '10 at 21:06

You can verify that your data is there by opening the backup drive, then go to /Backups.backupdb/YourComputersName/Latest/. This is a copy of the most recent backup of your disk. You should be able to view your home directory, or mount it in case you use File Vault.

Restoring is as easy as selectively copying files from your backup drive to your or your friend's machine. Also see here.

As for full system restore, this will require a partition on which to restore to. Which is major surgery, so to speak. You definitely want an unused partition for this. Since your friend probably only has a single partition on his Mac, this is not going to be easy. Partition resize utilities are available, but I've had a major commercial utility seen failing and destroying data on a Windows machine, so I cannot recommend that.

You can restore during Mac OS X Setup, and even just selectively e.g. restore Applications and User Profiles, if bad 3rd party drivers were acting up, for example.

Added information after third substantial edit of original question:

Only if your external Time Machine backup drive has a second partition on which you can install your system. Hope that your friend has his OS X system disk available, or prepare for that.

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Thanks, this is super helpful. What about the solution alluded to in the other answer and comment? Ie, can I plug the external drive I backed up to to a friend's laptop and boot from it, bypassing the laptop's harddrive altogether? – dreeves Oct 29 '10 at 19:33
@dreeves Yes and no. No because you cannot boot from Time Machine backup, use Carbon Copy Cloner or SuperDuper for that. You need another (preferably empty) external hard disk. Once you start Mac OS X Setup, you can select this disk as installation destination (you might need to format it first, but then it should work. Inserting the setup disk is non-destructive, you can just try it out and see for yourself). – Daniel Beck Oct 29 '10 at 20:11
@Daniel Beck, Thanks for setting me straight on this and for the pointers to those other tools! Realistically I'm not going to stay on top of this while my original laptop is still alive (unless I automate it to be as effortless as time machine). So am I on the right track with the addendum to the question? (My external drive is 4 times as big as my laptop's harddrive so there should be plenty of room to do this without another external drive.) – dreeves Oct 29 '10 at 21:00
@dreeves Somewhat. Problem is, how do you create another partition on a previously used disk with valuable data? There are tools for that, but I've seen them fail before, and it's not pretty. Big time data loss. You should be able to use two separate external disks, one where you install your system anew, the other provides your backup data. – Daniel Beck Oct 29 '10 at 21:15
@dreeves I assume your external backup drive is one big partition. The problem is how you fit another one (as system drive) on that. You either erase it (and lose your backups), or resize the backup partition (iPartition or something like that, have never used it though), risking loss of your backups in the process. The creation of a new partition itself is not risky, only how you make room for it. – Daniel Beck Oct 30 '10 at 10:56

Get an external USB disk and restore to that instead. That will leave the internal harddisk untouched.

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I'm already doing the timemachine backups to an external drive. Are you suggesting that I can plug that drive in to my friend's laptop and just boot from it, bypassing the laptop's harddrive altogether? I guess I could try that with my own laptop now. – dreeves Oct 29 '10 at 19:30
No. The time machine backup is not bootable. You could however create a separate partition on the backup drive and install to that. – Chris Nava Oct 29 '10 at 19:35
Cool, thanks Chris. See the addendum to the question. Do you have more ideas about the specifics of creating the bootable second partition on the external drive (without the original laptop, since I surely won't be prepared enough to have done this ahead of time)? – dreeves Oct 29 '10 at 21:10
@dreeves, no but you can attach a new, blank harddisk and your time machhine disk to your friends computer, boot on the OS X DVD and reestablish to the blank harddrive. – Thorbjørn Ravn Andersen Oct 30 '10 at 4:35

Answering the "steps 3 and 4" part, as others have covered the general principles:

Depending on how much you back up (and/or want to restore), there are several options:

  • To do a whole-system restore, hook up all the relevant drives and then boot from the install DVD. Once you get past the first screen, it'll have a Utilities menu with (among other things) Disk Utility (which'll let you create/erase a partition to restore to) and Restore System from Backup (self-explanatory). Do not proceed with the OS installation, it will have been recovered from the backup.

  • To do a more selective restore (or if you have the OS files excluded from the backup), install a clean OS from DVD, then when the Setup Assistant gets to the screen asking "Do You Already Own a Mac?", there's an option to transfer from a Time Machine backup -- you can use this to restore user accounts, applications, as well as some settings and misc documents. Note that you can also run the Migration Assistant after initial setup (it's in the Utilities folder) to do restores like this after initial setup.

They aren't as relevant to your question, but for completeness I'll mention a couple more restore methods:

  • To restore specific files/folders, use the Time Machine icon in the Dock to enter the cool spacey view-through-time mode; find the item(s) you want, press Restore, and they'll pop back into existence. Note that a few applications (I think mainly Apple's Mail, iCal, and Address Book) support this mode to restore their items without needing to know where they're stored in the file system.

  • Finally, you can open the Backups.backupdb folder and root around until you find what you need.

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Thanks Gordon, this is great info. Just to be clear, this can be done with a friend's laptop without creating a new partition on said laptop (the other answers suggest that it's a bad idea to do so), correct? Like I want to show up at my friend's doorstep with a laptop in several pieces, an intact external drive with time machine backups, and a deadline in a few hours and be like "quick, give me your laptop!" and have it acting like my own laptop, rest its soul, in a matter of minutes. – dreeves Oct 30 '10 at 0:47
Yep, as long as you bring a spare drive to restore to (as Thorbjørn suggested), hook up both that and your backup drive, and make sure you format & restore to our spare drive, NOT your friend's internal drive. After restoring, use the Startup Disk tool under the Utilities menu to select the spare drive, or just hold the Option key at boot and it'll let you select it then. – Gordon Davisson Oct 30 '10 at 1:20
I just edited the question to clarify it and moved the proposed steps to a separate answer. – dreeves Oct 30 '10 at 11:32

Based on initial answers (thanks Gordon, Thorbjørn, Chris, and Daniel!) it sounds like the following is a rough outline:

  1. Plug in the external drive with the Time Machine backups to the friend's laptop.
  2. Create a new partition on the external drive using the Disk Utility app that comes with OSX.
  3. Restore the Time Machine backup to that new partition.
  4. Boot up the friend's laptop from that new partition (hold down the Option key while booting and you should be given this choice).
  5. Voila, it should now be as if you're on your own laptop again.

Except I gather that step 2 should not be done while the external drive contains the only copy of your precious, precious data since it's risky to create new partitions. (For the same reason you wouldn't want to create a new partition on your friend's laptop to restore to.) So create that partition now, before your laptop is a smoldering lump. Which of course you need to do anyway to do your trial restore.

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You're almost there, but: how come you only have a single backup? – Arjan Oct 30 '10 at 11:40

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