What is Reddit's opinion of GLOTRENDS M.2 PCIe 4.0/3.0 Adapter Without Bracket for M.2 PCIe SSD (NVMe and AHCI), PCI-E GEN4 Full Speed?
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If I understand correctly, the 980 (non pro) is a PCIE 3.0 Drive. My adapter is this one: GLOTRENDS M.2 PCIe 4.0/3.0 Adapter Without Bracket for M.2 PCIe SSD (NVMe and AHCI), PCI-E GEN4 Full Speed https://www.amazon.com/dp/B079546NPR/ref=cm_sw_r_cp_api_glt_fabc_5M8EWM8HJX4QHS3QERAQ?_encoding=UTF8&psc=1

I am currently using the glotrends M.2 PCIe NVMe adapter . The amazon page for it claims full speeds, Gen 3.

I got the same results with a different adapter as well.

I have tried using the PCIe x4 2.0 slot above the GPU and the PCIe x16 2.0 slot below the GPU; both yield the same speeds.

Mac Pro 5,1 has two x4 slots and two x16 slots. All four are PCIe gen 2.

I this this is the one you have, maybe?

Do you have both CPU's installed? Based on this:

you need both use to all PCI-E slots. Though I think that is pretty standard for dual proc systems.

< $50

< $50

< $25