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The SN850 is a NVME PCIe x4 Gen4 and it should be compatible with the Rampage V M.2 x4 Gen3 port.

Have you tried to put the SN850 is a USB NVME enclosure to make sure it is working?

Have...

Motherboard Rampage V Extreme PCIe Layout:

PCIE_X16_1 (3.0)
-
PCIE_X8_2 (3.0)
PCIE_X4_1 (2.0) *shared with PCIE_X1_1 and USB3_E910 (ASMedia) and SATAExpress_E1 (ASMedia)
PCIE_X16/X8_3 (3.0)...

Only M.2 PCIe works on this motherboard.