GPS Question - Garmin Vista HCx. Wishing to load ShonkyMaps onto preloaded City

Greetings;

I've had my Vista HCx for about 2 weeks now. I managed to get ShonkyMaps installed via Mapsource after about 3 hours of fiddling and was pretty stoked with what this gave me.

Then I got the bug and ordered City Nav NT 2008 on MicroSD from the US for $200.

This now means that Shonky and City Nav are on different cards which isn't practical. I tried to load ShonkyMaps onto the new City Nav card and it keeps giving me an error about not being able to write to the requested memory space. I then tried to load City Nav from the device to Mapsource and it still didn't want to know.

Would I be better off selling the City Nav card and buying City Nav NT on CD so I can whack it into Mapsource and load it into the device that way? This way I can theoretically have Shonky and CN on the same card.

Or does anyone know what I'm doing wrong?

Thanks!!