3DS SD card transferring data HELP please! :3

Hi all, so I need help with transfering data from sd card to sd card. I know it's pretty easy to do on the computer, but I can't figure it out because I actually have 3 sd cards with data on them. Two of them are 2GB and do not have enough space for two downloadable games so I had to use two SD cards. But then I recieved a 12GB sd card and used that for awhile and now have a game downloaded onto it. I tried about a few months ago to simply copy the files from my other two sd cards and put the files on my 12GB sd card. When I start my 3ds up with the 12GB sd inside and go to play on of the downloaded games there is no save data.. The save game data is still on my other two 2GB sd cards, so I could always just have 2 sd cards just for those 2 downloaded games, and then my 12GB for actual use..But I don't really want to do that. Any ideas or help you can provide me with??? I tried backing up the games and then re-transfering the data, but alas it's still not working.