XPS 8910 How To Migrate System HDD to RAID 1 With Second HDD

I added a second SATA HDD to my XPS 8910, same size and model as System HDD.  Started up Intel Rapid Storage Technology (RST) in hope of migrating the System HDD to RAID 1, just as I had previously done on my old Windows 7 XPS 9100 using Matrix Storage Manager.  RST shows both drives, with the original drive labeled as "System", as same size in the Status window, but there is no Create button available.  If I add a third (smaller) HDD, RST then gives me the option to create a RAID 1 volume using the 2 non-System drives.

Why will RST not allow the System HDD to be migrated to RAID 1?

Looking at the BIOS, it's very different from my old system, which used to have a Ctrl-i capability to get into the Intel RAID options.  What is set there on the XPS 8910, just as it originally came, are the following:

Advanced

  SATA Operation   [RAID]

Boot

  Boot List Option  [UEFI]  (not accessible to change)

  Secure Boot  [Enabled]

  Boot Option Priorities

    Boot Option #1 [Windows Boot Manager]

    Boot Option #2 [Onboard NIC (IPV4)]

    Boot Option #3 [Onboard NIC (IPV6)]

Disabling Secure Boot would allow me to change Boot List Option from UEFI to Legacy, but I haven't tried for fear of messing things up, and I'm not sure it would make any difference to RST anyway.

Bottom line: How can I migrate my System HDD to RAID 1?  Can I do it without having to completely reinstall Windows 10?

  John