NVME M2 drives: which slot to use?

NVME M2 drives: which slot to use? pciex1_1 slot The slot description on my sabertooth Z77 motherboard are as follows slot 1 is PCIex1_1 slot, slot 2 is PCIe _1 slot (single at x16 or ask slot AdvancedOnboard Devices Configuration: PCI-EX16_2 Slot Bandwidth Runs at AUTO with USB3_E56 port and PCIEx1_1 slot

sgc4 slot Not sure why they didn't just link PCIEX1_1 to PCIEX4 as that slot usually becomes unusable due to the GPU also, i can't seem to find any