Try a new SD card, sometimes they just go bad.USB Loader knows the file structure it likes better than anyone else, so it might help to just have it make a new backup that you can try.
Try putting the USB stick in while empty and formatted FAT32 and make a backup of a game in the Wii CD drive.Again same as 3, make sure to back everything up. You could simply try deleting and recopying a fresh download of USB Loader GX onto the card first but I've found a full reformat makes a big difference. Click prepare game for emulation in Wii USB Helper then Play this game on PC.