I had this same issue in that I couldn't switch it to external. Step prior:
Smart Controller was off, inserted a 128 card, started drone and controller, then flew and recorded video. (Didn't check or change any settings prior to flight.) After flight, checked controller and all videos were saved to internal. Tried to move it to external with the controllers' app, but couldn't. Re-formatted external card using the controller app, powered off, then upon power up... controller detected card and asked if I wanted to save to external as default. (chose 'yes')