How can I make my computer believe a disk image is a hardware disk?
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:


Create your Disk Image (I've used a sparse bundle disk image).
Mount the disk image
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.