I have had this previously with 2012 and am experiencing again this morning. When I have an assembly open and I try to do a 'save as' it just sits there fat dumb and happy, me not so much. What I end up having to do is the close out everything that has any attachment to it, a drawing, a part, a higher level assembly that it is attached to, in order to actually do a 'save as'. Has anybody else reported this as an issue? I have never had that happen before.