The Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter lets you connect Thunderbolt and Thunderbolt 2 devices — such as external hard drives and Thunderbolt docks (not displays) — to any of the Thunderbolt 3 (USB-C) ports on your MacBook Pro.
Also it is bi-directional so you can connect Thunderbolt 3 Hard Drives and Docks to Thunderbolt 1 & Thunderbolt 2 Macs. Must be running macOS 10.12.5 or higher.
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