Alright, so I'm new to modding (like really new) and just added some materials and stuff to a cell (UnownedCell) and saved it and tested it ingame, it worked. When I started it this morning, I tried loading it into creation kit, it shows the stuff I did last night in the render window but when I try to save it over my progress I did last night, it says "File 'TestMod01.esp' is a master file or is in use. Please select another file to save to." I don't know what to do. Thanks. This is in the creation kit, btw.
