Quote from http://wiibrew.org/index.php?title=Main_Page :

1 May 08: A USB Mass storage device file/folder lister demo has been released on the #wiidev channel by svpe, the same person who made the first SD elfloader. It is still in testing stages and the code needs to be cleaned up, so no download link will be provided here. It is compatible with anything that adheres to the USB Mass Storage Device specification and FAT16, including USB memory sticks, flash drives, certain phones/MP3 Players, some FAT-formatted USB Hard Drives and even single-slot card readers. When a cleaned-up version is released it will be posted here. Very early and probably buggy source code is available now

http://repo.or.cz/w/svpe-wii.git?a=blob;f=usbstorage.c;hb=master