Thanks for all the great information here, hope you don't mind if I jump in on a slightly different topic.
I just got a HP EliteDesk 800 G2 SFF with Windows 10 and am planning to follow this thread for Mojave eventually. However, I want to use a 2013 MacBook Air PCIe SSD (Toshiba 256GB, apparently AHCI, *not* NVMe) in the 4X PCIe slot. I bought a PCIe adapter card on eBay and everything seems to fit fine (I get a red light on the PCIe card, not sure if that's good or bad but at least I know power is getting there). However I cannot see this drive in either the BIOS (updated to latest July 2018 version), or in Windows (Disk Manager only shows the Windows SATA disk). I've tried a bunch of BIOS settings, changing the PCIe slot, and disconnecting the SATA disk -- nothing works.
I've been bouncing around a lot of posts all over the place and have posted on the HP forum, just wondering if anyone here might be able to point me in the right direction. Many thanks in advance.