BambiBoomZ,
Chaos reigns on this topic..... there is so much to learn and so much mixture of correct and incorrect informaton. Please note that the comment "With the Z400, Z600, Z800 it seems that with updating to UEFI they can then can use M.2 AHCI" is incorrect. These workstations do not now nor will they ever have a UEFI BIOS. These will always only run on good old legacy BIOS, as will the xw generation of workstations. The Z420/620/820 workstations, however, can swing both ways.
What lets Brian run the Kingston HyperX Predator M.2 PCIe SSD as a boot drive in his Z600, in its M.2 to PCIe adapter, is the fact that its on-board controller is the AHCI type. Therefore it can be seen in the Z600 BIOS as a drive, and can be selected as the boot drive from there. Please see Brian's BIOS boot order picture HERE with that Kingston M.2 drive at the top.
We'll get this figured out eventually, and as always thanks for your excellent input.