I have a sparse disk image that I've been backing up to for a while, and the other day I thought I would see if I could boot from it just in case something went wrong. It is on an external FireWire drive. I don't know if this is the way to do it but I double clicked on it to mount the image. Then I went into system preferences/start up disks to choose it, and it wasn't a choice.
Maybe that's not the way to do it, I don't know.
Thanks in advance for any help or info you could give me.