Did you make sure you have a WBFS partition on there?
Firstly you need to make sure you have a partition that is free to be formatted into WBFS format by your USB loader.
Download EASEUS and resize your drive. Use EASEUS to make an area of unused space on your drive and then once its done doing that make that newly free'd space a partition in FAT32 format and make sure it is set to logical drive.
Stick the drive into the wii and then run ur USB loader, it should then try to format your new partition to WBFS.
Once that is done stick your drive in your pc and use WBFS manager 3.0 to load the drive and drop your .iso's on it from there or using your USB loaders DVD rip function.
This should work provided your drive is compatible.