I have a couple of BC files on my Micro SD for my HandHeld, and I just use a USB reader to plug into my PC and change the name of the chart I want to actually use to GMAPSUPP.IMG and the unit will read it, the one I am NOT using, I change the file name to the chart name so I know what it is, ie: Cheaspeake Bay, or SE Florida.

So you will only have to have one card but you can store multiple charts on it.