Well there you go. You formatted the drive for Wii U games, obviously it won't be detected by a Wii homebrew.
If you want to store both Wii and Wii U games on it you have to convert your Wii games to WiiVC format. Otherwise you can format the drive to FAT32 on a PC with guiformat to use it with USB loaders and other homebrew
http://ridgecrop.co.uk/index.htm?guiformat.htm (and optionally hide the drive with uStealth which will prevent the Wii U mode from asking to format it)
But that means you have to use another drive for Wii U games.