What version of Family Tree Maker are you using? The subject line says 2008, but that is way old.
You not NOT work on an Automatic Back Up, but work on your Working file. An Automatic Back Up is there "just in case". it will NOT include any Media file.
You can certainly keep the Automatic Back Up feature turned on, but it's more important to manually do a back up. I run a Tools, Compact File, which offers a Back Up, which I take each time. AND I do NOT save that Back Up to my C Drive, but to an External Drive.
Hope that helps,