Only one of my two NVME SSDs show up as bootable choices in my BIOS

I recently upgraded my machine, (new motherboard, CPU, and GPU). The only thing I kept was three SSDs: a 1TB M.2 SSD with arch linux installed; a 256 GB M.2 SSD with Windows 10 installed; and a 500 GB 2.5" SATA SSD which served as a games library for the windows install.

Also of note: I have a monitor with a built-in KVM switch which has this machine and others plugged in.

The new machine uses an ASUS PRIME X570-P as its motherboard. That board has the two M.2 sockets I need.

When I first assembled the machine, and powered it on, by the time I had gotten my monitor onto the correct input it was already booted into windows. I figured that I was doing things slightly out of order, but I proceeded to update drivers etc. in windows since I was there already (this included a windows update, as I have been waiting on my CPU and GPU for a while).

When I tried to re-boot into my linux drive, I was surprised to see the following in the SATA configuration menu:

M.2_1 Samsung SSD 85 (1000.2GB) 
M.2_2 Not Present

Which is confusing to me, because the 1TB drive is the linux drive, and when booted into windows the 256GB drive is correctly mounted as the C: drive.

I attempted removing the 256GB drive from the system altogether to boot into linux, however, it seems that the windows update clobbered the EFI system partition (at least) on my 1TB drive and if I try booting it alone, I get a blue-screen which reads something like “a reqired device is missing.”

I can fix my linux install (assuming it’s only the EFI partition which was clobbered by windows), but I’m at a loss as to how to get both drives to show up in the SATA configuration and to be able to choose which one to boot.

My SATA mode is set to AHCI.

The manual implies that there is a NVMe RAID mode setting but I haven’t been able to actually find it.

I’m a bit concerned that the motherboard decided to turn my drives into a RAID-0 or RAID-1 volume without my explicit consent.

Any ideas about what’s going on here, and how I might go about being able to use both M.2 slots simultaniously?

Thanks in advance.

here is a photo of the Advanced/SATA configuration screen from my machine, which is missing the NVMe RAID mode option which appears on page 27 of the manual linked above.

Is Legacy BIOS enabled?
Is the Arch disk BIOS or UEFI?
I’ve never had problems with Windows messing with my linux installs, so a educated guess is the mothernoard can’t see the linux install because it only sees UEFI drives not BIOS or the other way around.

That’s my educated guess, had a similiar issue like this and enabling legacy mode in bios options did the trick

I didn’t see such an option so far, but regardless the BIOS should show the slot as populated, even if the disk didn’t contain an OS at all

My Arch install is UEFI

CSM or Compatibility Support Mode was disabled, and it was set by default to secure boot only. Enabling CSM has not made the 256Gig drive show up in the BIOS, and has not made the 1TB drive visible from disk management in Windows.

Setting everything to “Legacy” I can now see all three of my drives (and all three become visible in disk management in Windows). As @Mariuspersen had suggested.

One interesting thing (now that I can see everything in disk management) is that my 256GB drive no-longer has an EFI system partition on it, and the 1TB drives EFI system partition is the windows boot manager.

Now I just have to figure out how to get Arch booting again without destroying my windows installation.

I have figured out why I only see one of my M.2 drives as present in the SATA menu. One of my drives is a 850 EVO, which is a M.2 form factor SATA drive, and the other is a 950 PRO which is a m.2 form factor NVMe drive. The NVMe drive does not show up in the SATA menu, it shows up in the NVMe menu (which I overlooked until now D’OH!)

The remaining mystery is why I currently have no EFI partition at all on my 950 PRO (where my windows 10 EFI partition should be), and how the windows EFI partition got moved to the 850 EVO, overwriting my linux EFI partition.

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