Hello Cyracus,
Indeed, it could be running in sata mode. As i have seen in some videos, this problem could be solved (note: the video was using a Z97 board) by going into Advanced\Onboard Devices Configuration and changing option "PCI Express slot bandwidth" from "Auto Mode" to "M.2".
I tried doing so but this option is missing in my bios, as i stated before i already updated my bios so i dont know why this is missing. Maybe i should contact Asus support.
Cheers