The ASUS PN51 keeps the same two-drive storage layout as the PN50

The ASUS PN51 sticks with the same storage layout as the PN50 — one M.2 2280 PCIe 3.0 x4 slot, one 2.5-inch SATA III bay. ASUS didn’t add external SATA ports on this generation, so what you see is what you get: two drives, one fast, one flexible.