I've read there are compatible 2.5" HDDs that use the power from the Wii, but I need to find one that doesn't power down whilst idle...
I just use a WD ext with it's own power supply...they tend to be more reliable than others brands and USB powered types.
Using a 1TB (NTFS) for my Wii atm
Only use full clean images, got loads of stuff that I don't even play myself, plus NGC images.