install.img from hard drive rather than boot.iso?

[Bruno Wolff III]

In order to work around the current dbus issue I copied my repo mirror
to a spare hard drive on the machine. I strip out some of the stuff in
the image directory to save space, but I found that the install wants to
use the install image file from the repo on the hard drive rather than
the one on the boot disk (boot.iso).
Is there a good reason for this?