This is what I use on my DS1621+ https://www.amazon.com/gp/product/B09NMSS4QB/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 uses a single USB port and both bays are addressable to the NAS (USBShare1 and USBShare2). Or you can use the hardware R1 switch on the USB Enclosure to do a hardware mirror over USB.