Sunday, 14 December 2008

Can I restore a trashed disk using a wireless Time Machine backup?

Mark Wheadon made an interesting comment on my Wireless Time Machine post the other day. He wanted to know if it would be possible to restore a trashed Mac from a wireless SMB Time Machine backup by pointing the OS X DVD Disk Utility at it.

I don't think this would be possible (please correct me if I'm wrong -- I'd really like to know), as using an SMB network share relies on setting the TMShowUnsupportedNetworkVolumes system preference. This is turned off by default and I suspect impossible to change when you've booted from the OS X DVD.

Instead, the approach I take is to have two separate backups -- one a full disk backup that just clones the entire disk; and separately, a wireless Time Machine backup.

The cloned disk backup is great when your disk dies, especially if you use similar hardware to the actual disk in your machine: if you get a failure, simply swap the hardware and carry on. I use Carbon Copy Cloner to make my cloned backup -- it's free, it's certified and it has an option to start automatically as soon as you attach your backup drive. Combine this with a 2.5" SATA disk in a USB enclosure and you have a drive ready to take the place of your MacBook's internal drive within a couple of minutes.

However, making a clone requires you to attach a physical drive, something that is too easy to forget doing regularly enough. It's almost guaranteed that the one day you forget to clone your disk is the one day your disk crashes irretrievably... This is where the wireless Time Machine backup comes in. Once you've got it set up, all you have to do is leave your Mac turned on and attached to your WiFi and you will get regular backups of your recent files (up to once an hour if you want).

So the answer to Mark's question would be that if my hard disk crashed, I would swap it out with the clone and copy any recent files from my Time Machine backup. This would deal with the immediate term -- generally the most crucial time when you have a disk crash. I could then look at restoring any other files I need from Time Machine at my leisure, though I would take a clone of the clone as soon as physically possible. I might also turn off Time Machine temporarily so that the out of date clone doesn't use up unnecessary space on the backup image -- but this would have to be a choice based on how long before I'd switch to using the clone as my main drive.

I'm not sure whether it's possible to use Disk Utility's Restore function from a Mac that has the TMShowUnsupportedNetworkVolumes preference set to true. Has anyone tried?

4 comments:

Kevin Peter said...

According to this web page (see "Time Machine Full System Restore" under section 7), it is possible to mount a time machine backup share and restore a trashed Mac.

According to this page, you can boot up, hold down the "c" key, go to the terminal in the Utilities section, and mount the share via the command line. Their example shows mounting an AFP share, but I imagine that it can be done with a Samba share as well.

It doesn't say whether one has to do anything with the TMShowUnsupportedNetworkVolumes option, but you could try that option via the command line in the terminal too.

I haven't tried this method, so I can't verify how well it works. Once I get my time machine backup share working, I think I'll give it a try.

Adam Cohen-Rose said...

Thanks Kevin -- this looks very hopeful. If anyone has tried it, please comment on your experiences!

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Charles said...

You can restore from an SMB network volume after booting from the DVD.

You need to run the Terminal app from the DVD, enter the TMShowUnsupportedNetworkVolumes command

You need to create a mount point where your backup share can bemounted. Do this via the terminal:

mkdir /Volumes/Backup

Next, manually mount your SMB share where your .sparsebundle is located via the mount_smbfs command line.

ex: mount_smbfs //user:password@server/share /Volumes/Backup