When I mess with a mod, save it, exit the CS, start Oblivion to test it, close Oblivion, and reopen the CS to continue messing with my mod, but I can't save it. I get the error "File is a master file or is in use." I've selected it as the active file so I can edit it and see it in the Cell view. If I don't select it then the building isn't there. I figure I'm doing something wrong, but I can't figure out what it is.

I've tried saving it as a different name, but when I have to load it so I can work with it the same thing happens. So how do you guys save/reopen your mods when you are working on them?