Basically every Wii has it's own set of "keys". These are basically long strings of numbers (and letters?) which are unique to that Wii. These keys are in the NAND dump, and if a Wii has a NAND dump restored which has different keys, it won't boot, because it's built to only boot stuff with it's own key. I think. And in anticipation of your or someone else's future question, yes the keys in a NAND dump can be changed so a dump from one Wii could be used in another, but it is a lot of effort, you have to disassemble the NAND dump, change all the keys and reassemble it (or something like that). Bushing will do this for a small fee, but I doubt anyone else in the Wii "scene" really has the expertise to do it/could be bothered. It is a LOT of work, I believe.