The Asus P4540UQ 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 for installation. (The M.2 slot supports SATA only Solid State Drives)