Your motherboard spec page URL is:
http://usa.asus.com/product.aspx?P_ID=lco8LZWTqWIhdLmv
Look on page I-22 of your manual - installing SATA drives. Make sure you aren't using SATA ports 5 or 6 - they support SATA/RAID only.
In the BIOS, under Storage Config, choose SATA mode - Make sure it's on SATA.
I don't think there's an IDE setting in your BIOS. This is seen in some BIOS and yes, should be set to IDE and not AHCI - but usually doesn't mention SATA. The AHCI setting allows special SATA features but has to be turned on before OS is installed - don't worry about this.
Check the SATA wiring - I think you may have it on the RAID ports.