When using Audio Surgeon you may encounter an onscreen message about not being able to write to an external drive. 

Your Mac OS does cannot write to some external drives because of the formatting used on these drives.  This is not an
error within AS it is and issue with Mac OS and its functionality.  If you do a search about "Mac OS cannot write to external drive"
you will see a bunch of results about this.  Here is one and it discusses ways to overcome this issue.

https://www.makeuseof.com/tag/solving-the-read-only-external-hard-drive-problem-on-your-mac/