M.2 is basically just the name for a small port that uses the PCIe lanes, which is much faster than SATA. SATA has become a bottleneck for SSDs and pretty much any decent SATA SSD will be bottlenecked by the SATA connection these days, so they made what is called "m.2" ports to basically bypass SATA and go directly to the PCIe lanes. I know m.2 is a funny name and its so funny its kind of confusing. There is no m.1 or m.3 so I have no idea why the weird name.
Separate from m.2, there is Nvme.
Nvme is the protocol that replaces SATA protocol (AHCI).
There are m.2 drives that use Nvme and m.2 drives that do not use Nvme. Any m.2 drive that uses Nvme will only work on a motherboard that supports Nvme, which has become a standard feature now but a few years ago there were some motherboards with m.2 ports but no Nvme support and this lead to a lot of confusion for a year or so with these drives. m.2 drives and matchboards that do not have Nvme support still use the SATA protocol which is slower, but not as slow as using the SATA protocol over the SATA connection.
If you are buying a new motherboard and a new m.2 drive they will almost for sure both have Nvme support so there should be nothing to worry about.
SATA connections top out at about 600 mbps.
m.2 connections that still use the SATA protocol (AHCI) can reach about 2-3 gbps.
m.2 with Nvme is basically as fast as PCIe, which is way more than any m.2 drive uses right now.
When you see M.2 followed by some numbers, like m.2 2280; the "2280" references how long the drive physically is. For a desktop they will all be the same. Sometimes there are shorter ones for laptops but for a desktop you don't need to worry about that part at all.
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Last edited by UltraMega; 06-20-2019 at 10:24 PM.