Synology DX517 5 Bay Expansion Unit

The Synology DX517 5-Bay Diskless Expansion Unit (DX517) lets you expand your Synology DiskStation NAS by up to five additional hard drives.  The  Synology DX517 expansion comes with five disk bays compatible with 3.5" and 2.5" SATA HDDs and SSDs, one eSATA port, and two 80mm x 80mm x 20mm fans.
 

DiskStation compatibility

Add more drives for your NAS to accommodate more data or to enhance RAID capabilities.  The DX517 is compatible with the DiskStation DS1517+ and the DiskStation DS1817+.  It syncs automatically with the host NAS once attached, allowing seamless management via Synology's Disk Station Manager (DSM) software.