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In this case, Intel built the machine to work with M.2 SSDs, adapters Intel did not validated.
Now, it works fine on the NUC6i3SYK and it does not work on the other NUC, most likely this is a compatibility problem, even though both PCIe are similar the chipset is different on each of the NUCs. Perhaps a contributor who was able to set up this configuration can share with us the model of the adapter.