As simply as possible RAID enables the user to utilise 2 (or multiples of 2) as either 1. mirrored, where all data is written to both drives, whereby you have a permanent mirror (if one goes down, you have the other) OR 2. where alternate bytes are written to first one, then the other drive (so effectively increasing read access by up to 50 % (I have forgotten the term for this second method) but this means you have only half of any data on one HD at any one time.
1st method is for where you want peace of mind, 2nd method gives you a little more speed.
Clear as mud?? perhaps somebody else can explain it better.
TOK