Hi shadowsports,
Thanks for taking the time to post some information against my question.
I had already done some research and also got some feedback from Gigabyte Technical Support and came to the conclusion that I would need to re-install Windows 10 after first setting the SATA mode to RAID. This was based on my inability to to get both the M.2 drive to work at the same time as RAID. For example, the SATA mode was set to ACHI by default and while in this mode I could boot into Windows 10, installed on the M.2 drive. If I changed the SATA mode to RAID (Intel RST Premium With Intel Optane System Acceleration) the PC wouldn't boot as the OS couldn't be found, presumably because it wasn't recognising the M.2 drive (for some reason). Now, this behaviour only started after I tried using EZ RAID in the BIOS. As I explained, the very first time I changed the SATA mode to RAID I was presented with the option to press ctrl I to enter the configuration utility when the machine was rebooted, however, I chose not to use this as I wanted to use EZ RAID. Big mistake!
Tonight, just before I trashed the Windows 10 install on the M.2 and set the SATA mode to RAID, I decided to try resetting the BIOS to factory defaults. After doing this, I set the SATA mode to RAID and, hey presto, I got the "Press ctrl + I to enter the configuration utility" option displayed on boot. This time I followed the prompt and was able create a RAID 1 array on the two Seagate HDD's. The utility did recognise that these two drives had been part of a RAID 1 array, however I had to delete it and then re-create it. Not a problem as I have backup images of all my drives/partitions. On starting Windows, this allowed me to format and partition the new drive.
So, it appears I have acheived what I wanted, however, based on your original response and on the insistence by Gigabyte Technical Support that this was NOT possible without first setting the SATA mode to RAID and THEN installing the OS, I'm a little confused and concerned about exactly what I have!
Am I getting the best performance from the M.2 drive? Why is it working, when Gigabyte TS said it wouldn't? Is it likely to cause problems down the line? I believe you alluded to this in your post.