Back in the middle of 2015, I wrote an entry on sorting out NVMe as the next way to connect SSDs to your system. Someone I know online was recently reading it, and he mentioned that he'd never heard of the 'U.2' connector that I talked about in that entry and that in fact I described as the way that future NVMe SSDs would be connected to your machine. In an aside in that entry, I wrote:

(PCIe and thus NVMe can also be connected up with a less popular connector standard called M.2. [...]

In 2015 and even 2016, U.2 was a reasonably big thing; you can read one example of it (and see some pictures of a U.2 connector and U.2 SSD) in places like this April 2016 article. Since then it has quietly vanished away, swept away by that 'less popular' M.2 standard that I mentioned in my aside. In fact the Wikipedia page on U.2 is pretty amusing (at least right now) due to its 'U.2 compared to M.2' section, which comes across pretty much as the last cry of people who cannot stand to see their beautiful thing crushed by the marketplace. U.2's fade out evidently didn't take all that long either, since when I did a late 2017 entry on sorting out M.2 and NVMe, it wasn't even on my radar and certainly didn't show up on any of the motherboards I looked at at the time.

(U.2 is apparently still sort of out there, especially in datacenter applications, and it seems to show up every so often on semi-consumer motherboards. See eg here.)

Technology change and failed standards are not exactly new to the PC world, but for me this is still an interesting and impressive example of it in action. U.2 was the obvious thing in the middle of 2015, and then two years later it had just disappeared completely.

(Possibly even in 2015 U.2 was more hype than anything else and I was taken in by it when I wrote my entry.)

Sidebar: Some speculations on why M.2 won out over U.2

I can think of a number of plausible contributing factors:

• M.2 was easier to put into laptops, because it required fewer parts (eg no cables). That gave it volume and we all know volume drives down price.

• M.2 was more flexible, since an M.2 connector can be used for either SATA or NVMe. Both manufacturers and consumers could trade off cost versus performance without having to change anything other than the M.2 card in use.

• Most people don't use more than one or two drives in their computers.
• Most people prefer the simplicity of plugging a M.2 card into a motherboard connector rather than mounting a separate drive and running cables to it.

• Intel didn't make enough PCIe lanes available in consumer chipsets to run enough U.2 drives to be attractive. Whether U.2 or M.2, Intel was always only going to give you one or two.

• M.2 SSDs (whether NVMe or SATA) are cheaper to make than U.2 SSDs.

While U.2 theoretically makes it easier to have larger NVMe SSDs, my impression is that in the consumer market the largest limiting factor on SSD sizes is how much people have been willing to pay for them. This certainly is the case for me.

(In the enterprise market I've read stories saying that the limit is how much data loss people want to be exposed to from one device failing.)