Dissatisfied with the Wii’s onboard storage? You’re not alone. Luke Plunkett from Kotaku found this tip at Gizmodo, which cited Lifehacker’s Kevin Purdy. And now here it is on MyThings.
Ordinarily the Wii can only read 2GB cards. To effectively double your storage space, load a 4GB card with 2GB of dummy files after formatting it with SDFormatter, insert it, and then delete the dummy files.
Read the full, detailed explanation here.