The standard file system used for Wii homebrew and USB loaders is FAT32. Because FAT32 does not support single files larger than 4GB, a standard Wii ISO cannot be copied to it directly. WBFS files automatically trim the fat, dropping most games well under the 4GB limit.
If you have acquired a raw game dump or downloaded a file in a different format (like Dolphin's compressed .rvz format), you will need to prepare it for your hardware. Wii Roms Wbfs Europe
For the few games that are naturally larger than 4GB even after being converted (like Super Smash Bros. Brawl ), management tools can split a .wbfs file into two parts (e.g., .wbfs and .wbf1 ) so they fit seamlessly on a FAT32 drive. How to Convert and Manage European Wii Games The standard file system used for Wii homebrew