Intel® NUC Kit 5i5RYH is already compatible with M.2 PCIe NVMe storage boot. However, it is necessary to use M.2 PCIe NVMe drives, instead of using regular 2.5” x15mm or HHHL Add-in card SSDs with adapters.
Intel® 750 SSDs are not available with the form factor M.2 yet. Samsung already launched to the market the model SM951-NVMe compatible to the NUCs.
Samsung SM951-NVMe (256GB) PCIe SSD Review
This NUC model requires with BIOS version 0246 or later to be compatible with M.2 NVMe SSDs.
BIOS Release notes
I think the problem may be that the 750 series SSD does not route the CLKREQ# pin out to the PCIe connector (pin 12). It seems that the NUC may require this signal. Assuming this is true, I would like to understand why, and if it is possible to disable this requirement. My reasoning is that I have an m.2-to-PCIe adapter that does have this signal routed out to the PCIe connector, and it works fine on the NUC. Both the Intel 750 series and another SSD of the same type (card dimensions and PCIe x4 connector), which also does not have pin 12 connected to CLKREQ#, do not work (on the NUC). Can anyone tell me if this is plausible and what I might do to get around it?