I'm trying to do some research on TimeMachine right now. I don't want to purchase an external hard drive, just to learn about some software, and so I'd like to be able to tinker with the settings, by just creating a disk image...

But when I mount the disk image, TimeMachine doesn't see it as something it can back up to. So I'm wondering how I can lie to the system, and make it believe that the image is in fact real.


  • So, just to be clear: The disk image you are creating and want to experiment with is stored on your one & only internal hard drive? Jun 28 '13 at 16:18
  • It doesn't matter to me. It could be on a network device, or on another partition of my one and only drive. I just need to experiment for monitoring purposes. :) Thanks! Jun 28 '13 at 18:55

Aloha, counterbeing - I think we've worked together before!

I like the previous answer that creates a partition, but if that doesn't work for you, try this:

  1. Create your Disk Image (I've used a sparse bundle disk image).
  2. Mount the disk image
  3. From the command line, enter the following:

sudo tmutil setdestination /Volumes/{mounted-image-name}

• Start Time Machine manually within the menu bar via "Back Up Now"

If all goes well, you should then back up to the image. It will need to be large enough to contain all of your backup data - or Time Machine will likely refuse to use it.

  • 1
    Hey Scot, funny running into you on the web. I'll give this one a shot. It sort of side steps the question a bit. But it would get me to the end goal of the question. If there is a solution that actually accomplishes the goal exactly as described (because such a technique would be useful in other circumstances) I will probably move the credit over there. But if this works, you get the points! :D Thanks Scot! Jul 3 '13 at 16:07
  • +1 Works good. Is there any performance disadvantages for doing it this way? Jun 14 '14 at 15:24
  • This has come in really, really handy for my current situation - I normally network TM backup to OS X server but the box died. While waiting to get it fixed, I plugged the server's external disk into my mac and can now perform a TM backup to the same disk image my mac usually uses via a network share :)
    – Spyder
    Aug 11 '16 at 10:24
  • This still works in 2019 on Mojave.
    – Kerlix
    Oct 16 '19 at 7:53

I don't know of a way to do this, but if you have enough space left on your drive, you could add a small, temporary partition instead of using a disk image. This creates a real partition, and can be done without erasing current partitions by resizing them.

Note: If you are partitioning your boot drive, you may need to run Disk Utility from either your install disk or the recovery partition for some versions of OS X. At least 10.8.3 allows you to do it directly, though.

First, open Disk Utility from the /Applications/Utilities folder and click on your hard drive in the sidebar (the disk, not the partition). Then, go to the Partition tab and click the + button. Click the new partition and enter an appropriate size and optionally rename it using the fields on the right. Then press the Apply button and wait.

When you are done testing with your partition, you can remove it by clicking on it in the Partition tab and pressing the - button. Then, drag the bottom of the other partition down so that it fills the free space left behind and press Apply.

  • I would consider this route if I had another backup of the data on the only external I'm carrying with me (I'm currently traveling). However, I've been burned before, with complete data loss, and would rather not manipulate my partition table. Though, I have had some success using tools such as iPartition, I think I'm just going to let this one lie. Thank you though! It is a viable solution, just don't want to do it. Jul 3 '13 at 16:05

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