Actually the data and error checking is striped across all three unless you have a rare controller which supports raid 4 <img src="/images/graemlins/smile.gif" alt="" />

Another option is to buy a pair of (or more) servers that are just plain simple servers, no redundant anything and just let the servers themselves be redundant. This works well if you don't have the budget for servers with redundant power supplies and raid controllers and such, just set up some scripts to keep them matching and if something fails in one you run from the other.