When you say the M.2 PCIe card is installed in the short slot, are you referring to the PCIe slot nearest the CPU? This is pci slot 1, which although physically is a 4x pci lane slot width, only 1x pci lane is active - the M.2 drive will NOT work in this pci slot. The M.2 pci adapter card must be installed in a pci slot with at least 4x active PCI lanes, and ideally PCIe 3.0 compliant, e.g. either slot 4 or 5, (since slot 2 is being used by your GPU).