I [had] an Asus A8N motherboard myself. From what you quoted, Pete said the Raid configuration didn't really help with game speed, so if Raid is not that big of a deal for you, then I would disable it in the BIOS. Since my computer died a terrible death a few months back, I won't be able to walk you through this, but I do have a general idea; when you boot up, hit F12 (or F10) and mess with the configurations that come up for Raid, and more specific for turning Raid off. After you do this, when you reboot the PC, and let the onscreen text go, a message for a separate Raid configuration comes up on boot with the A8N motherboard and says to press F6 (I think) on boot to change the Raid status, but I don't know if this is necessary if you change it in the BIOS first. Anyways, fiddling with these should get rid of your warning message. The reason you are getting that warning is because one of your drives not mirroring properly - perhaps it is going bad? One last thing... on my A8N motherboard, I had to set my Sata drives on the Raid 1 through Raid 4 ports, which were separate from the standard Sata 1 through 4 ports (there are 8-sata ports, but 4 are four Raid), so you may have to move those drives to the standard Sata ports. Sorry I can't be of more help. I hope you find my knowledge useful.
"The true value of a human being is determined primarily by the measure and the sense in which he has attained liberation from the self." -Albert Einsetin