MS server 2008 OS installation & RAID issue

[Nigejk]

I have an N54L with 3 disks installed

slot 1 - 500GB

slot 3 - 2TB

slot 4 - 2TB

I have installed MS server 2008 on the 500GB disk succesfully. I then via the RBSU set the two other disks into a RAID 1 configuration. I get two disk errors after POST and when continuing Windows fails to boot. 

Do I need to set up the disks as RAID prior to installing the OS or somehow make the single disk RAID. I do not seem to make any progress on this.

Any help appreciated 

NB I have gone back to IDE in the RSBU and set up the disks as a mirrored drive. Will this be the same as setting up RAID in the RSBU or is their a  performance penalty going it this way?