RST on Windows 10- building a RAID-1 volume, can't de-select either disk as "soure"

[buckyswider]

Hi there, I have a system that had a RAID-1 volume out of two identical M.2 disks.  We had a no-boot problem, and the vendor came out and replaced one of the modules and everything came up OK.  That was *really* weird- why did the system not boot with only one bad module?  (If in fact that was the problem, which I'm dubious of).

Now I noticed that the RAID group is no longer functional.  I had one of the modules in a "failed" Raid volume and the other was non-RAID- the non-RAID being the "System" disk. So I deleted the failed RAID volume to try to re-build a RAID-1 config.  Create Volume, and select both disks.  However, there's a radio button for each drive, asking which one is the source (apologies, I'm not near the machine right now, so I don't have the exact verbiage).  However, they are BOTH greyed out, so I can't de-select either.  I did not proceed, because I want to ensure the proper drive gets selected as the source.

When I deleted the RAID volume, the Disk Formerly Known as Failed RAID got mounted as the F: drive in Windows.  Is that possibly why it can't be de-selected?  If that is the case, what do I need to do to re-initialize that drive to make it available as a fresh target as the mirror for my system drive?

Thanks!

(p.s. I looked at this config because of a nearly identical problem with a twin machine- question on that one being posted in a minute)

[buckyswider]

I couldn't figure out how to tell in BIOS which was the boot disk, but I took a leap and assumed the one marked "System" in IRST was it.  So I used Disk Management to unmount the former RAID disk which was mounted as "F".  I  then went through the volume build screens again, and I hit "next" after the screen where it wouldn't let me de-select either disk to "keep" the data, and on the next screen it only gave me one option- delete data on the non-"System" disk, which is exactly what I wanted to see.  Checked that box, hit go, and VIOLA! An hour later I had a fully synced RAID-1 boot drive!