RST RAID-1 disks now "separated" after MOBO replacement- system won't boot-which is boot disk?

[buckyswider]

Hi there, had a non-boot situation on a RAID-1 volume of 2xM.2 disks.  In the BIOS, the volume shows as "failed" and both drives as "error occurred".  Vendor settled on it being the MotherBoard (?).  Offline diagnostics said both disks were fine, but "UEFI Hard Drive" was not even visible in the Dell BIOS Setup menu.  After the vendor replaced the MOBO, the UEFI Hard Drive was back showing again, but the system won't boot into windows- giving some sort of corruption error.  Looking at RST in the BIOS, the RAID config is now showing with the RAID volume "Failed" with only one physical module in it, and the other module as a "Non-RAID" device.  

What I want to do now is to try to boot whichever device the system is NOT trying to boot right now- if it's booting off the RAID volume, switch it to the non-RAID disk, or vice versa.  However, this is my first rodeo on UEFI- I can't find a way to switch from one to the other.  There's simply a single entry for "UEFI Hard Drive".  The only thing I can think of is to pull one of the drives and try, and if that doesn't work, replace that and pull the other and try again.  Is that the only way to try that with UEFI?

And I'm wondering if this could potentially be caused by attempting to boot up without selecting RAID mode, as much as I implored him to do so.  We did switch it to RAID, but still got the boot errors.  Thanks!

[buckyswider]

I was never able to figure out the answer as to which device was the "boot" device.  I ended up disabling the devices one at a time in BIOS and then trying to boot.  The former RAID disk was logical toast- when the other one was disabled, leaving that as the only disk drive, the BIOS didn't even list a "UEFI Hard Drive".  So I physically removed that one, and (of course) the disk that had been removed from the RAID volume still wouldn't boot.  So I booted off a Win10 recovery stick, and went into the windows prompt and through a combination of Diskpart  and  bootrec /rebuildBCD I was able to boot back into Windows again.  Yay!  Once we're all up and running again I'll put the second m.2 back in and try to create a RAID-1 volume again.  

Not out of the woods just yet as we can't connect all the users to their microsoft accounts yet due to the mobo swap, but that's a post for another forum...