The most likely cause of this issue is your computer's security settings regarding mounted volumes.

Unfortunately, if you deny access at any time that setting is retained by the operating system until you manually change it in System Preferences (you will not be prompted to change it).

To give ShotPut Pro (or any other application) access to Removable Volumes, go to 
System Preferences > Security & Privacy -> Files and Folders.  Once there, give ShotPut Pro the proper access.

These steps are outlined in detail here