I just use a file manager (well cp/mv) on my etrex 30. Are you sure you meant to buy the etrex 10 and not the 20? It can't accept sdcards or maps.
I have a very simple usecase for a GPS. All it has to do is give me a speed display, and log a track to GPX of where I've been. That's enough for me. My old eTrex H did both of these, but had the habbit of cutting out randomly. So I flogged it, and got the 10 instead. As it's a whole generation beyond the old H, it seems it's more advanced, so I was curious what else I might be able to do.
Dropping GPX files into Garmin/GPX/ did the trick.
Cheers
J