SSD – Does M.2 M Key Motherboard Connector Always Support NVMe?

NVMe support on the motherboard is only about the firmware (BIOS/UEFI). That's because NVMe is not a physical connector or electronic transmission protocol like PCI Express. Instead, it is a way to talk to the SSD controller, much like AHCI is way to talk to the SATA HBA (or backwards-compatible PCIe SSDs).

It is also only about booting. After that, the operating system takes over.

With M.2/U.2, there are two types of connections:

• SATA (supported by U.2 ports and M.2 slots keyed B or M)
• PCIe (supported by U.2 ports and M.2 slots keyed A, B, E or M)

A SATA M.2/U.2 SSD interfaces with an external (not on the SSD but on the mainboard) SATA HBA (Host Bus Adapter) using the SATA protocol. The operating system typically talks to the SATA HBA using AHCI. Everything works the same as when connecting a SATA drive to a SATA port. With U.2, you are even using a regular SATA cable.

A PCIe M.2/U.2 SSD on the other hand comes in two varieties. It has an equivalent to the SATA HBA built-in. The host can talk to this controller using one of two standard:

• AHCI (none available with U.2, but still possible)
• NVMe

An AHCI controller has limited performance. However, it offers the best compatibility because virtually all (but very old) firmware already supports AHCI. As such, you can boot from such SSDs no problem. (Provided the SSD itself isn't somehow limited in that regard.)

The NVMe specification is more suitable for non-rotational drives like SSDs and offers better performance by reducing overhead and whatnot. However, until relatively recently, mainboard firmware did not support this standard and could not boot from NVMe SSDs. You could still access them from your operating system, provided appropriate drivers were available.

Wikipedia has a great image on this topic:

(Just ignore that it's apparently only about SATA Express/U.2, M.2 is mostly equivalent in this case.)

tl;dr: All NVMe SSDs use a PCI Express connection. As such, they appear the same to the BIOS, whether connected via U.2, M.2 or a full-size PCI Express slot.

However, having an M.2 or U.2 slot does not guarantee in any way that the firmware can boot from NVMe storage devices.

The manufacturer could provide a NVMe-boot-capable firmware later, yes.

There are no dual-personality SSDs. Source: Current offerings.
That's a given, of course, because it would be very complex to build a controller that connects to all necessary pins on the M.2 connector. The controller would also have to support running in both a SATA and PCIe mode. And last but not least, the controller would have to detect which connection to make. AFAIK, sensing is not a part of the M.2 connector, however.

So, while technically possible (probably), it won't ever be economical.

SAS is also entirely unrelated because it is not used in consumer-grade PCs. There would be no benefit in providing SATA/PCIe connectivity for SAS devices.

It's also NVMe vs AHCI and SATA vs PCIe. The only way to find out what protocol and connection the SSD uses is to look at the specifications.

A PCIe SSD could use whatever proprietary protocol the manufacturer decides on.