If I open a Rhino file, I can save it perfectly well. But If I open a file and then open Grasshopper, I can’t save the 3dm-file because Rhino says it is in use. I can only save it as a new file (Save as).
I checked the directory where my 3dm file is and a 3DM_TMP file is created (by Grasshopper?) that presumably prevents me to save the file.
I played a little with it and the way I get it to work is first open a new Rhino document, open Grasshopper, then open the Rhino file I want to work on, and now it saves fine.