Since Android 4.4 ("KitKat"), external apps are not allowed to write on any external storages (External SD-Cards, USB-Storages, etc.)
For Photo Mate, this means you may view and even edit images on that storages (when you edit, the XMP-Data is stored in the internal Photo Mate folder), but exporting images to that location is not possible. What if you want to save to your external storage, anyway?
If you store your files in the external app directory, make sure you backup this directory before you uninstall the app! Android will delete this folder when you uninstall Photo Mate!