Hi there!
Could you check on whether Audiobook Builder is allowed access to removable volumes? Here's how to peek at that:
1. Go to the Apple () menu at the top left corner of your screen and click the System Preferences... menu item. The System Preferences window will appear.
2. Click the Security & Privacy icon.
3. Click the Privacy button in the row of tab buttons near the top of the window.
4. Scroll down the list on the left side of the window and click the Files and Folders item.
5. Scroll through the list on the right side of the window and see if Audiobook Builder has a Removable Volumes item. If it does, make sure that has a checkmark.
Let's knock that out as a possibility and take it from there!