I only receive notices of these postings in a weekly summary and see that you asked your question almost a week ago, so probably you already have your answer. But, if not, I am actually a bit surprised to see kirupa’s recommendation,since his credentials are far superior to mine.
What I would say is that the way you have phrased the question suggests that you are using an absolute URL or Native Path with File to open a directory into which you wish to save state. The File API provides support for some special URL protocols which are – in my experience – guaranteed to give you access to the preferred location for such content, irrespective of OS or OS release: File.applicationStorageDirectory.
I don’t know why a user would set permissions on that sub-directory that would prevent creating of a new file there, or modifying an existing one, but, if they did, your application would get status telling you that and you could use a dialog to let such a paranoid person choose an alternative location. Using any sort of file:// url with an absolute path, or a Native Path string to the same effect, causes you to have to write protective code based on Windows versus Mac OS Sierra, so I think the opaque solution built into AIR is a better one.