You cannot simply drag and drop raw ISOs onto a USB drive and expect the Wii to read them. You need management software to structure the archive correctly. Wii Backup Manager (Windows)
To ensure your console recognizes a full archive of games, they must be organized in a specific folder structure on your USB drive or SD card. wii wbfs archive full
Use Wii Backup Manager to convert, split, and transfer the files. You cannot simply drag and drop raw ISOs