If I understand correctly, your city nav maps are not installed on the computer, only on the Nuvi, but you are somehow able to view them on the PC when it is connected. Using Mapsource on the PC to make routes on maps that are not actually installed on the PC but only being "viewed" from the Nuvi would seem to be the problem to me.
When you click the upload icon, there's nothing to upload. It's like you were playing with a "virtual" route. No substance.
'09 Wee Strom