Do I need to switch SATA to RAID mode?
thas the first thing you do when you want to enable the cache in Intel RST's software. Just set the SATA to RAID - don't bother with which raid you should run, nor should you create a raid array.
I did look for the iGPU setting but I could not find it before. (I blame EFI BIOS ) I looked again and found it in the system agent configuration area.
So i take it that you have the iGPU taken care of?