I bought a 73sv non chirp when cabelas clearanced them out over the winter. It's registered in express along with my 2 93sv chirps. The build date is late 2015 according to the unit. I went to do my free map updates and it only let me download 1 map. I thought "no big deal, I'll just copy it onto each SD card and I'll be golden. It worked for both of my chirp units but the 73sv said it could not unlock the map. Any ideas?