You've tried with wii backups in WBFS Manager???
If the Wii-backups doesn't load properly to a drive in WBFS Manager, then u should probably:
Reinstall WBFS Manager / Reformat your drive / Or try this app instead: http://wbfssync.codeplex.com/
If its just Gamcube-backups:
(I'dont know if there is a way to plug a WBFS-drive in to a Gamecube-console) :S
But even if u got the Gamecube games to that WBFS-drive, it would not work when u plug it in to the Wii cause it would either
freez or crash upon launching.
Anyhow, there is now way to make Gamecube backups run through USB on Wii (yet), but when the day comes that someone find a way, I would like to be the first to know! :P
Or if u already know a way, please... please share.
I do believe though that u can launch GC-backups with a "Gecko" or an "SD to GC-memorycard adapter" and launch them that way... I've ordered one for myself yesterday cause I don't own a GC-memorycard and thereby cant save any GC-saves
on my Wii... But if I find a way to make it work i will post a comment in this tread about it.
I hope u found your answer.... And I wish u luck. :)