The Asus P1501UF Laptop Supports 1, 2 or 3 Drive Configurations. (One 2.5" Solid State Drive or Hard Drive in the Primary Drive Bay and/or One M.2 NGFF (2280) Solid State Drive in the M.2 Slot and One 2.5" Drive in the Modular Bay) Note: The second 2.5" Drive replaces the optical drive when installed. If installing the second 2.5" Drive in replace of the optical drive, you will need the Modular Bay p/n 04150-00030000 for installation. If the 2.5" Drive was not installed from Asus in the primary drive bay, you will need the 2.5" Drive caddy p/n 13NB0FD1AM0101 for installation. (The M.2 slot supports SATA only Solid State Drives)