The DS920+ has two M.2 NVMe slots for caching and an eSATA port for expansion
An eSATA port supports one DX517 expansion unit (5 more bays), and two M.2 NVMe slots provide SSD caching. There’s also one USB 3.0 port for external storage.
An eSATA port supports one DX517 expansion unit (5 more bays), and two M.2 NVMe slots provide SSD caching. There’s also one USB 3.0 port for external storage.