Why do you have cIOSX rev20 installed? Here at wiihacks, we recommend cIOS38 rev17, because some disks don't seem to work properly with other revisions. If your disk is FAT32 formatted, then you might need to use cIOSX rev19, but there is no need for rev20. Also, make sure you are using the lastest stable version of USB Loader GX. You can get it by using the Easy installer. Even then, USB Loader GX might not work. If so, try another USB Loader, like Cfg USB Loader or wiiflow.