Unable to save changes to files in Visual Studio 2013

For me it was Google Drive. Once I've closed it, I was able to save the file.

Try running Visual Studio as Administrator.

Extensions might be the problem. Or you could be opening some readonly files. It's worth to mention that some of the extensions can turn any file you create in to a readonly file, and this would be the same result that you're facing.

So first, disable all extensions, and check for the little icon near the files's name (in the tab) that indicates a readonly file.