I think we're getting closer to understanding why some M.2 PCIe SSDs will allow booting from older workstations and some won't.... see my post HERE which details that there are M.2 AHCI SSDs and M.2 NVMe SSDs. To my knowledge the Z600 motherboard that Brian has his M.2 AHCI SSD/PCIe adapter booting from is not a UEFI motherboard, and the ZX00 and xw HP workstations all run only "legacy" BIOS.
Nelson, I had no idea that the Samsung 950 M.2 SSD had the two types of BIOS built in..... so it might be able to work with the proper PCIe adapter in the Z400/Z600/Z800 in addition to possibly also in the xw workstations too. Interesting.....