The HP ZBook 17 G6 Mobile Workstation Supports up to 5 Drives. (One 2.5" Drive in the Primary Drive Bay and/or Three M.2 NGFF (2280) Solid State Drives and One 2.5" Drive in the Optical Drive Bay) Note: The primary 2.5" Drive and two of the M.2 slots are accessible under the service door. The third M.2 slot is on the back side of the system board and is only accessible by removing the system board. If installing a 2.5" Drive in the optical drive bay, you will need the ODD 2.5" Drive bracket carrier p/n L71657-001 (In bracket kit) for installation. If the primary 2.5" Drive was not installed from HP, you will need the 2.5" Drive primary drive bracket p/n L303394-001 for installation. (All three M.2 slots support SATA and PCIe NVMe Solid State Drives)