I have this exact problem. If I import a mesh from D;, it can’t be reimported. If I import it from C;, it can be. If I move a mesh back over to C:, and retarget the source file, I can reimport it. I think my project started on C as well.
It’s aggravating how UE4 wants everything to be stored on C; since so many people are using smallish SSDs these days.