When I was testing out USB sticks on my NDS recently I discovered you need to format it the right way to make it visible on NDS. Even if I formated it to FAT32 it didn’t show up. Problem was I was formating a level below top level on the USB-stick. And the top level was not visible on my Macbook. Once I made it visible and formated the top level as FAT32 and “Master Boot Record” it all worked OK.