a follow-up:

My Asrock X99 board has 10 SATA3 connectors. RAID is supported on 6 of them. I had my two SATA drives on the non-RAID connectors. After I moved them to the ones that support RAID, I was able to install the Intel RST drivers. (Which replaced Microsoft drivers.)

I don't see a world of difference in drive performance, but thanks for the information.