As I understand it (and this is only what I've read), the Original SD card (for non Pro-Users) which you have to use, has a finite amount of licenses on it pre-installed. Each license is used up at each successive map update, and (apparently) once you use the final license, that's it no more allowed.
Not sure how they would enforce this on the Pro-Nav, unless the same license system is already installed on the HDD ?