Is RAID thoroughly disabled in BIOS? Is there a chance that any RAID0 remnants remain present anywhere on any M.2 devices?

I have RAID enabled and when I try to disable it, I do not boot at all.
This is strange, because I am not using RAID. The machine (MSI Titan GT80S SLI) came in a RAID configuration, which I undid.

None of the M.2 devices have RAID on them.

A likely consequence of a broken RAID0 device, depending on what the alternative option(s) is/are.

Undid according to what instructions? Do you understand what the RAID0 it shipped with means?

What makes you sure? Are not at least two of them the same M.2 devices it shipped with? Did you completely wipe them and install everything from scratch?

I don't know what you mean by a broken RAID device?

Nothing is broken on the system. The alternatives to RAID is AHCI I believe. It does not boot in AHCI mode.

The machine came with 2 SSDs in a RAID 0 configurion, which I undid. I copied the data from both disks onto 1. Then removed those disks. The data on disk 1 is now fully mirrored, well cloned, once a week, onto disk2. They are both PCIe.
One disk holds everything 2 disks held. Then it was copied over to the other disk. Anyway, this has nothing to do with why non-PCIe / NVMe disks do not appear in the bootable devices list in BIOS. I have 2 disks that are visible and 2 that are not.

I have not tried installing Linux on one of the PCIe devices. It should probably work. The system just doesn't appear to be capable of booting off SATA.

When switching from Raid to AHCI you need to re-install OSes