ok this is a potentially stupid question , but i have just set up my server with centos. However at the time of set-up it only had one drive , i now have another drive i can easily slot into the machine as i want to set-up raid 1 on it. Do i now have to do a complete OS re-install so that the new second drive is also running centos. Im thinking yes but was hoping raid would magically do it for me :)

I'd make a backup to start, one that can be accessed. Either turn it into a virtual machine or image or tar it or something. Then I'd bet there are plenty of how-to's either on RH or mirrored on Centos or Fedora already on how to do it. I doubt you can just slip it in and let it build the array. Guess you could do that for a partition on the current drive and part or all of the second drive but anytime you mess with boot area and hal you have to work. "Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10