I have had a similar problem with my new Asus A8N-SLI Deluxe on the nvidia raid controller. I know this is not your problem but it may help. This raid controller comes with 2 drivers, "NVIDIA RAID CLASS DRIVER (required)" nvraid.sys "NVIDIA NForce Storage Controller (required)" nvatabus.sys In order to pickup the raid array (instead of 2 independant drives, as per your problem) I had to load, "NVIDIA NForce Storage Controller (required)" FIRST then load, "NVIDIA RAID CLASS DRIVER (required)" afterwards (pressing S to add one after the other). Once I did this I could partition my 140gb raid 0 array and install properly. Hope this helps! For all you slipstreamers, I guess this would also apply, making sure the NForce Storage Controller is loaded before the Raid Class Driver.
