Connecting M.2 drives to various things (and not doing so)

Summary

As a result of discovering that (M.2) NVMe SSDs seem to have become the dominant form of SSDs, I started looking into what you could connect M.2 NVMe SSDs to. Especially I started looking to see if you could turn M.2 NVMe SSDs into SATA SSDs, so you could connect high capacity M.2 NVMe SSDs to, for example, your existing stock of ZFS fileservers (which use SATA SSDs). The short version is that as far as I can tell, there's nothing that does this, and once I started thinking about it I wasn't as surprised as I might be. What you can readily find is passive adapters from M.2 NVMe or M.2 SATA to various other forms of either NVMe or SATA, depending. For example, there are M.2 NVMe to U.2 cases, and M.2 SATA to SATA cases; these are passive because they're just wiring things through, with no protocol conversion. There are also some non-passive products that go the other way; they're a M.2 'NVMe' 2280 card that has four SATA ports on it (and presumably a PCIe SATA controller). However, the only active M.2 NVMe product (one with protocol conversion) that I can find is M.2 NVMe to USB, generally in the form of external enclosures. (NVMe drives are PCIe devices, so an 'M.2 NVMe' connector is actually providing some PCIe lanes to the M.2 card. Normally these lanes are connected to an NVMe controller, but I don't believe there's any intrinsic reason that you can't connect them to other PCIe things. So you can have 'PCIe SATA controller on an M.2 PCB' and various other things.) When I thought about it, I realized the problem with my hypothetical 'obvious' M.2 NVMe to SATA board (and case): since it involves protocol conversion (between NVMe and SATA), someone would have to make the controller chipset for it. You can't make a M.2 NVMe to SATA adapter until someone goes to the expense of designing and fabricating (and probably programming) the underlying chipset, and presumably no one has yet found it commercially worthwhile to do so. Since (M.2) NVMe to USB adapters exist, prot...