There are two sets of SATA data and power connectors at the back of the unit where the main processing board is attached. Drives are installed by simply stacking them one at a time into the cage and mounting them using the included screws. These are designed with rubber grommets in the sides of the case to reduce vibration and noise. There is a thermal probe on the main board and the manual recommends sandwiching it between the drives, using a small piece of included tape to attach it to the top of the bottom drive. This controls the operation of the fan at the back of the unit.
With both drives in let's reassemble and power on the NAS 4220.