Here are the rules for you to use an external USB hard-Drive.
1- Be sure to have the latest Wii firmware: (To have your wii actualized)
2- Format the USB hard Drive to FAT32
3- Max 120GB (Though some website claims that a 300GB hard drive works too)
4- Remove Any SD card form the SD slot
The Wii should detect your USB hard drive as an SD card and you'd be able to use your External hard drive as storage (Virtual console, Game Saves...) just as it was an SD card.