Is it at all possible to setup Time Machine so that it backs up to a disk image on my external Mac?

Here's my setup:

  • My MBA which will be backed up
  • MBP which is connected to the wireless network
  • External drive connected to the Mac which I want the data to be backed up to
  • MBP is running Snow Leopoard
  • MBA is running ML

Is this possible? I could also connect this drive to my router, but as of right now, it doesn't look like it's seeing it.

  • So, you want to back up to a disk image on the external drive? – Nathan Walker Jul 20 '12 at 0:12
  • Yes, or just the drive itself, which would be simpler. – Charlie Jul 20 '12 at 1:03

This tutorial should be able to help you:


I just tried it and it worked for me. Not sure how reconnects will go after a reboot or something, but it shouldn't be that difficult to get working again.

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  • Thanks for the link, problem is, using defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1 on ML doesn't show any "unsupported" volumes, unless it's just not seeing it. – Charlie Jul 20 '12 at 2:04
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    On Lion, I had some issues getting this to work as well. Make sure you connect to your shared folder, then close and reopen System Preferences before trying again. Since Mountain Lion isn't out yet, I obviously wasn't able to test this on that OS. – Nathan Walker Jul 20 '12 at 2:28
  • So I should add the folder to sharing on the MBP, then connect to it on my mac (via AFP?), then run the terminal, then reopen system prefs and setup the time machine from the MBA? The thing about these guides is that they aren't always clear as to which computer I should be doing what on. – Charlie Jul 20 '12 at 2:32
  • Add the folder on the External drive to sharing on whatever Mac has the drive connected, then connect it to your other by using the Shared section on the Finder and opening the folder from the shared list. Then, reopen System Prefs and the shared folder should appear under the "Select Disk" option. – Nathan Walker Jul 20 '12 at 14:14
  • It works!!! Like you said, I'm not sure how reboots will work. Basically, my MBP (with the disk) turns off at 3:00 AM and then turns on at 7:00 AM, or something like that, so the disk would be unavailable during that time, but so am I (sleeping). But, I might need it to connect to the drive automatically when my MBA boots (wakes up), do you know of anything that can do this? – Charlie Jul 20 '12 at 18:18

There is the tmutil setdestination command that you can use in Mountain Lion, even when the GUI is not showing the unsupported drive.

Stole the answer from this blog post:


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