Fat file system is 8.3, with 8 characters reserved for the filename and 3 for the extension. When you name a file longer than that, a virtual file name is created and a the real name is different. When you named it "MOON (U).NDS", the first part of the file is less than 8 characters long, so the real name ended up with a space in it. Same is true of "Contra 4.nds" If you had named it "MOON (US).NDS", It would likely have run fine.