Guys, seriously... just burn your games onto DVD for backup. They are very cheap.
That way, if your external HDD goes out, you'll have them always to restore.
No need to mirror or anything like that.
By the way, I have about 80 games on my 250 GB hard drive (scrubbed and update partition removed). I have pretty much every game that isn't shovelware (i.e. Bratz, Imaginz, Nickelodeon or Disney games, etc.) - AND every single WiiWare and VC game as of a couple of months ago - so consider that when choosing a size.
2 TB seems like overkill for a Wii only HDD.