SN750 PCIe 3.0 4x but 5 GT/S only

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You need to give us some more info to go on, like:

  1. What’s the make and model of your motherboard
  2. Do you use an M.2 to PCIe adapter card?
  3. Publish some benchmark test results (Crystal disk on Win, or fio on Linux (live)).
  4. What is the low level format of your SSD (512B or 4096B)?