I see this wifi back up app and stuff, but I remember an old homebrew that you start, it asks you to swap to retail card and it stores it in ram, then you swap flash card back, and it saves it to SD. I haven't messed around with a DS in a while (4 yrs+) and I'm guessing it was tossed because some games are larger then system ram. info here's and idea, do a block based transfer, load retail into memory until certain point. save block to sd, then swap again and continue at previous point (maybe small overlap for merging check). I'd use the GBA slot version but my sis has DSi XL, no slot, well since my mom has original DS (fat, it was cheap and shes a cheapskate) be nice if someone continued the old homebrew
yes they did have that but it only worked on the fat ds and because of firmware revisions and changes in the ds carts it was quickly layed to rest