there is a pretty comprehensive list here:
i have a few LaCie portable drives that i use to back-up files on my PC.. the slim 2.5" style ..both 160GB and 250GB both of which are NOT recognized by the wii.. i thought it might have something to do with the fact that these type of drives
are fully powered via the usb cable.. maybe if i get an external drive that requires an external power source (plug into the usb socket AND a power supply unit/power adapter) THAT MIGHT work? but i am not sure..
i'm currently looking at the list to find a compatible drive and i will go grab one of those.. see if it works any better than these LaCie drives!
hope this helps?