TIG. Many thanks for the info.

I am using Snow Leopard v 10.6.8 and have full read and write permissions for my complete hard drive. I have tried to save this local collection in my Documents folder as well as the hard drive / library /application route.

Many thanks to Dave R, Krisidious and Jeff Hammond for the in-depth answer.
It was a shadow thing. I followed your advice and my problem is solved. I had 2 vertical section cuts through the same room in a house model, each facing in the opposite direction and each one captured on it's own scene. All the parameters for each scene were identical. As I would toggle between the 2 scenes I would get "white" walls / "gray" (shadow / shading) walls effect. It was rather daunting.

If you are looking to create terrain to model on, you can turn on your sandbox plugin, draw some quick topo lines (even better if you can get topo lines from an existing map/gis website) and then simply hit the 'from topo lines' tool in the sandbox tool kit and whalla you've got your site topography to model your deck, patio, & landscaping on. Sometimes I prefer to model the landscaping (paths, walls, etc.) in a plan view, then elevate the lines directly over the sandbox topography and use the drape tool to place the lines I need on the generated slope. Lots of tutorials I'm sure on how to use the sandbox tools.