The Lenovo ThinkPad T420 Laptop Supports up to 3 Drives. (One 2.5" Solid State Drive or Hard Drive in the Primary Drive Bay and/or One mSATA Solid State Drive in mSATA Slot and One 2.5" Solid State Drive or Hard Drive in the Optical Slim Ultrabay) 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 slim ultrabay for installation.