I have been biulding PCs for around 8 years now and most ofthem have been raid (0) stripe ,I allways buy Gigabyte boards ,but the principles are the same . however without having the manual for your board in front of me I could well guess way off the mark .
first of all why do you have your os on a single drive and then attempt to create a mirrored raid array (1) , this would not protect the drive with the OS on ,only what data went on to the array .(duplicated) .
in effect your setup would create (if you could) an OS on the 250gb ,and keep 500gb for data ,backed up (duplicated )
some golden rules are to format the drives to be used individually before creating the array ,if its for data only make sure the array is not made bootable.
I have never used Raid(1) ,prefering to use
Raid (0) for speed ,I must have Biult 40+ PCs in the last couple of years and most on Raid(0) ,never lost a drive to date.
be patient and write down whatever you try ,when I first started I often spent hours going over the same things ,and not seeing the obvious .
sorry I cant be of more help at the moment , If I have time later I will download the manual for your model and see what is possible ,there are so many raid chips out there these days and often methods are different to achieve the same result.
good luck . post back of you solve it ,it may help others in the same predicament .