The Lenovo ThinkPad W520 Laptop Supports up to 3 Drives. (One 2.5" Solid State Drive or Hard Drive in the Primary Bay and/or One mSATA Solid State Drive and One 2.5" Solid State Drive or Hard Drive in the Optical 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 2nd HDD ultrabay enhanced media bay tray p/n 43N3429 for installation.