I... have some trouble understanding what you're asking, but files that are imported end up wherever your unreal engine project is (because they need to be converted to something UE understands), which I guess is somewhere on your C drive in your case?
Unreal Engine uses a Cache to store temporaly your materials and meshes. You can change this folder-destination to your other drive under Edit->Editor Preferences and then under General->Global->Local Derived Data Cache.
