Yeah, seems like all of them can be recovered as recovery tool like that one might be able to 'revert' the file where scandisk has failed to sort em out properly. Now, its just a guess that if you have, let's say "Somegame.nds" which is, lets say 128megs, and you have a .chk thats also suspiciously so 128megs in size, just renaming it back manually should also work. Its a theory to me really as I never had this problem but in case you didn't delete anything, try renaming stuff back to .sav or .nds or try that tool (or at least keep it around in case it happens again).