This is mostly extra notes to my recent separate threads about how to UEFI-boot OpenBSD in various ways. The following OS-unrelated factors added ambiguity when trying to figure how to UEFI-boot.
I started with a particular machine from 2013. Whatever I tried, it would just not boot off a PCIe NVME SSD: I'd install the OS on the SSD, and the installation process worked out perfectly, but it wouldn't boot, neither in UEFI/GPT nor in legacy/MBR/BIOS mode. I tried with using an USB memory stick with the OpenBSD boot loader on it as trampoline for the boot process. Interestingly, the MBR boot loader *would identify* the SSD *but could not read* from it. The UEFI boot loader (BOOTX64.EFI) would both identify the SSD and could read on it! Hence we're seeing a system with weird one-quarter-support. How unintuitive is that. While figuring my way through that problem, I stumbled on mentionings that at least two PCIe NVME SSD models apparently include legacy ROM:s to provide AMD64 (and I think ARM64) BIOS legacy disk IO on a system where the BIOS otherwise would have no PCIe NVME SSD support at all. Primary models with legacy are Samsung 950 Pro [1] and Intel 750. This could be useful for getting a higher-speed SSD on an older computer for someone. Samsung ditched the legacy ROM thing quickly though so Samsung 960 and other newer SSD:s will not have it [2]. The drive I got going by the UEFI USB boot disk trick, is a Samsung 960. For older machines, there may also be ways to patch UEFI firmware to support PCIe NVME SSD:s [3], didn't try though. Tinker [1] https://web.archive.org/web/20161027233731/http://www.samsung.com/semiconductor/minisite/ssd/downloads/document/Samsung_SSD_950_PRO_White_paper.pdf [2] https://us.community.samsung.com/t5/Memory-Storage/960-Pro-Evo-Boot-Support/td-p/66895 [3] https://audiocricket.com/2016/12/31/booting-samsung-sm961-on-asus-p6t-se-mainboard/
