Synology RackStation 4 Bay Expansion Unit (Synology RX418) - Drive Bays: 4, AC Input Power Voltage: 100 V to 240 V AC, Power Frequency: 50/60 Hz, Single Phase, System Fan: 40 mm x 40 mm x 3 pcs, External Ports: 1 x eSATA Port, Applied Models: RS820RP+; RS820+; RS1219+; RS819; RS818RP+; RS818+