The DS1520+ adds 10 more bays via eSATA and has a PCIe slot
It supports up to two DX517 expansion units (10 additional bays) over eSATA, and a PCIe 2.0 x2 slot accepts a 10GbE network card or M.2 NVMe adapter.
It supports up to two DX517 expansion units (10 additional bays) over eSATA, and a PCIe 2.0 x2 slot accepts a 10GbE network card or M.2 NVMe adapter.