Whenever you use a RAID controller, the Windows installer will need the proper driver to access the RAID, or it will simply not see it.

That is why they set it up as RAID by default, even with a single drive, as this way Windows has both the RAID driver and NVMe driver available during boot.

As long as you did not configure a RAID volume, you can freely switch between RAID and NVMe access.

RAID volumes are not recommended for consumer systems, and you will also loose the ability to monitor the drives with SMART.

@Vanadiel​ Thanks. I wanted to use RAID1 to avoid downtime if a drive fails. It's unfortunate that windows setup doesn't recognise RAID volumes, in spite of it supporting them in windows.

Anyway, just in case anybody has the same problem, there is a way around it in this post.

From online forums, that's how I got the driver in Terminal

      ./SetupRST.exe -extractdrivers SetupRST_extracted

Thanks for that .. I wonder why all Dell agents I talked to didn't come up with an answer, when it's all there. 

Re:  I wonder why all Dell agents I talked to didn't come up with an answer, when it's all there. 

From what I understand, Dell agents are trained to provide sales support and warranty support.  They are not trained to show people how to use computers.  To receive answers for your questions, they must open a case for warranty support if you have hardware issue.  Or, they would provide answers if you request a paid software support.

Re:  Anyway, just in case anybody has the same problem, there is a way around it in this post.

With consumer class systems, you should be able to setup hardware RAID for your SATA drives with onboard controller.  Using Windows storage space, you can set up RAID volume for NVMe SSD but for storage only. 

How did you manage to install Windows on RAID 1 with NVMe drives?  Does your R16 storage controller support NVMe hardware RAID?

The R16 storage controller supports RAID. But it's a combination of software and hardware using Intel RST.

A true hardware RAID would be expensive.

R16 specifications

As I mentioned earlier it's always set to on by default, so you get the RAID driver added to your Windows OOB image.

However, no RAID volumes are configured so you can really only create a RAID 1 with it. How to do that is explained here

To my knowledge any other RAID besides RAID 1 will need a reinstallation of the OS because I know of no way to convert an existing image into a RAID volume.

@Chino de Oro​ My problem was windows installation stopped recognising the two SSD's paired in RAID1 when system crashed after Dell automatic BIOS update. So, either setup BIOS to NVMe or it won't see the drives at all. That's even after a complete erase. They spent hours online and over phone and couldn't tell me simply download xyz drivers for RST  and RAID available in Dell website, and use with Win setup. That's not a big ask, especially when problem was caused by BIOS automatic upgrade.

Anyway. That's why we have these community chats so we can help each other, and thanks for the advice received. 👍

Interest to know if the R16 can support NVMe RAID before installing operating system.

In short .. yes .. 

BIOS set to RAID (which by the way is the default when PC came from Dell). Added the RST drivers to the windows setup USB drive (can have it on another drive and swap, but I didn't want to mess about), and windows recognised the drives, and continued the setup. I'm arguing that such drivers should, like all others be included in the windows setup anyway, or that computer sellers should include them on their websites, or Intel should have the drivers, not the windows RST Optane setup which only works from inside windows (how to get there in the first instance ?!) available.

I hope my journey and frustration would help somebody else out there.