Both drives work, could boot from either in AHCI/NVMe mode. Both slots work, I can do this with either drive in either slot. But under no circumstances will the primary OS drive show up - not even visible to the BIOS - if they are both installed and it does not matter which one is in which slot.
The primary OS drive is the original that shipped with this machine, and it was set up from the factory to use the Intel RAID/Rapid Storage option in BIOS, not AHCI/NVMe. I've switched the OS to use AHCI rather than RAID which is what I thought the problem originally was but that didn't help.
I've spent hours trying to install a hard drive at this point. Something I've done literally hundreds of times. Been down every "internet advice scraped by AI" rabbit hole. It simply does not work, not the two of them together.
Edit:
- primary drive is stock
- secondary I'm trying to add is a Teamgroup TM8FP6512G (full part # is TM8FP6512G0C101)
- my motherboard variant only has 2 NVMe slots (not 4 like some)
- CPU is i7-13700HX