OWC Gemini is a versatile way to add storage to small Macs. It allows two 3.5" or 2.5" hard drives/SSD's that can be addressed separately or configured as RAID 0 or 1, has an SD card reader, two USB 10 Gbps ports, a 2.5 Gbps Ethernet port and an HDMI port for video/audio pass-through using the Thunderbolt 3 connections. There are two Thunderbolt ports, one connected to the host and the other for daisy-chaining additional devices. It can also work on a PC, although it does require a Thunderbolt connection to enable all the other functionality. This unit has seen little actual use, and is in good physical condition, although it somehow got some scuffs along one top edge. Comes in original box with all the accessories including thunderbolt cable and power brick.