Hi Fozzie, if the SATA controller mode is set to RAID in BIOS, all drives connected to this chipset will be in RAID or RAID-ready mode and should give you the same advantages as with AHCI.
The possible cause of your issue could be related to compatibility, since older drivers for this chipset do not support Windows* 8.1 and newer drivers do not support older chipsets. You could try Intel® Rapid Storage Technology 11.7 but you are probably missing the Intel® Chipset driver and Intel® Management Engine driver, due to the chipset / operating system compatibility mentioned earlier.
https://downloadcenter.intel.com/Detail_Desc.aspx?DwnldID=22194