The M.2 x4 connector is dedicated to M.2 NVME/SATA SSD. There is no support or guarantee that any other cards can be used.
I too would like to see support in place so cards like this could be used to implement a NAS (etc.), but it's just not going to happen (they refuse to do the (unfortunately *huge* amount of) additional validation that would be required.
Yes, I am sure - and your reference means absolutely nothing. Yes, of course it's true that there are x4 lanes of PCIe into this slot.
The issues are clear. First, there is no guarantee of BIOS support for the PCIe bifurcation needed to support how these lanes are used (i.e. individually or as a groups). Second, due to the intended use of this slot being to exclusively host a M.2 SATA/NVMe SSD, there is no guarantee of support for things like downstream OPROMs (hosting graphics video driver, for example). Finally, more of a statement than anything else, these NUCs include support for Thunderbolt and this is the intended path for the connection of external devices in these designs.
Bottom line, no guarantees of any kind. Something might work... or... it might not. Further, since these NUCs are all discontinued at this point, there is absolutely no chance of any BIOS updates that might improve this picture. Feel free to try anything you would like. If the results are suboptimal, well, all you can do is move on.
Sorry, reality bites,
no guarantee is so much better than how you put it before - you cannot
this thread caught my attention because I have the same NUC and use a thunderbolt eGPU enclosure. I have a faulty nvidia card which I needed to run some diagnosis software (mods/mats based on linux) that didn't detect the PCIe through thunderbolt so I bought a m.2/NVME adapter to PCIe 16x riser (designed for bitcoin mining due to only having 1 lane) and while I was waiting for it, I read this thread and got a bit concerned.
Well, the part arrived today, shoved it into the M2 slot and bam works a treat! so definitely no guarantees but for anyone googling, it worked very well for me!