“Mirroring” (aka RAID level 1) is where a computer has two or more identical drives, and anything that gets written to one is also written to the other(s). This provides a basic level of protection should one of the drives fail. But there are many, many reasons why it’s not a viable backup method, especially for a commercial website. At a minimum, any backup should be physically separated from the computer itself, preferably at a significant distance in case of fire, flood, or similar disaster. I’ve been hearing a lot of radio advertisements for a remote backup system called Carbonite. Also, PC World has a list of 17 online backup sites. If you’ve tried one of these I’d like to hear your opinion on how well it works.