I'm obviously missing something in how BIOS determines which disk to boot with UEFI. I have a dual boot setup with two SSD disks. Disk0 has Arch, Disk1 has Windows10. Ordinarily I use systemd boot on Disk0 to boot Arch or Windows.
I needed to upgrade Windows10 and it fails in dual boot, so I disconnected Disk0. The system didn't have any trouble booting into Windows, so I finished the upgrade and restarted a couple of times just to make sure the upgrade had completed.
I then put Disk0 back in but Windows10 loads directly and I don't have a UEFI boot option in BIOS for Arch on Disk0 any longer.
Removing Disk1 still doesn't allow booting from Disk0.
How does BIOS detect a valid UEFI partition and why wouldn't it now detect the partition on Disk0?