I'm storing backup of my Gmail account in a disk image, to cut on number of files on the main HFS+ volume. This makes Time Machine ignore the existence of this volume, just because it's in a disk image.

Is it possible to get Time Machine to treat the disk image as a regular disk, and get Time Machine to back it up as if it were a "physical" volume?

This is on Mountain Lion 10.8.2, on late 2009 white unibody Macbook.

  • You can’t back it up as a physical volume, it will back up as a dmg file. Not sure, really.
    – duci9y
    Oct 31, 2012 at 17:36
  • I was asking precisely because I'm interested if I can force Time Machine to treat this volume as a physical one. I'm only separating it to make HFS+ happier and snappier. Nov 6, 2012 at 16:56
  • I have the same question, but with different goals - I'm trying to convince Time Machine to create a local snapshot on APFS so that I can get some test data.
    – Hakanai
    May 8, 2018 at 2:12

1 Answer 1


To back up a disk image, you back up the .dmgfile itself, instead of the virtual disk the system shows you.

However, unless the disk image is a sparse bundle image. Time machine will have to backup the entire disk every time the disk changes.

  • Can this be done while the disk image is mounted? If so, how does it avoid problems with the image being modified during the back up process?
    – Michael
    Feb 12, 2015 at 18:57

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