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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.

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You can’t back it up as a physical volume, it will back up as a dmg file. Not sure, really. – duci9y Oct 31 '12 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. – Ivan Vučica Nov 6 '12 at 16:56

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.

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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 '15 at 18:57

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