Hi!
Just to be in sync, the most current RAID drivers are found at the following URLs:
ESRT2 RAID driver for Windows*
Hence, if you selected ESRT2 as the onboard RAID controller card in 'AHCI Capable RAID Options' BIOS section, then, when prompted to manually load up the RAID driver, you find a the ESRT2_Windows_signed_DRV_v16.03.2014.1127\windows8_ws2012_x64 path, within your USB thumb drive. If RSTe is rather chosen in the mentioned BIOS section, you’ll find it at intel rste 4.1 gd windows package\Drivers\x64\Win8_2K12\AHCI.
Once you upload the RAID driver and finally see the volume to install Windows on, you remove the USB thumb drive, click on the Refresh button, and click on Next:
Hope that helps!