RX418 4-Bay Expansion / 1U Rackmount / 1 x eSATA / 3 Years Warranty / Optional Rail Kit: Sliding (RKS1317), Mounted (RKM114) Supported Models RS1219+, RS820RP+, RS820+, RS819, RS818+, RS818RP+, RS816, RS815+, RS815RP+, RS815, RS217* *Supports creating volumes across different units ONLY.