I've just got my own DS Lite, and am surprised to see that you can't save your files on a separate memory card. I can't really understand the logic of it, but anyway...
Researching this, it seems the only way I can keep my save files without the game is to download illegal ROMs, which is something that I have no interest in at all.
Please can someone tell me how I can solve this problem. It's almost as if Nintendo WANT me to break the law!! All I want is to keep my save files. Help!