Yea, this model could be SU4 or SU5 originally. The one I opened was SU6. Problem is, resaving doesn't work. The bug follows the model. But I need to use that model.
I got around it though by making a script that found the missing materials, and made fresh new copies of the materials, then iterate over the model and replace the old buggy materials with the new ones. That preserves the UV mapping that was done.
I guess this is one of the one timers kind-of bug.
I did have problems with the same model earlier. Due to the network paths. But only when I had V-Ray installed. V-Ray would try to access the original bitmap, but the path doesn't exist any more. So everything I did anything in SU, it'd freeze for 30secs. I sorted it by writing a script that'd export all textures of networked materials and reload them from a local path. Though, at that time I didn't notice I was missing some textures.
weird stuff. and I'm stuck with this model. ghnngh