To get the best results from your External Graphics Development Kit be sure to keep the following in mind:
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Hot-plug and un-plug of the External Graphics Development Kit requires returning to the Login screen i.e. you can only plug / unplug when logged out or shutdown. This requirement is the initial implementation of the External Graphics Development Kit while it is a developer-only feature.
There is also no "eject" button or other means to safely remove the External Graphics Development Kit once logged in.
If you plug in the External Graphics Development Kit while logged in, you won't be able to use it although there will be Metal Device for it. In this case there should be a macOS notification to log out and back in to enable the External Graphics Development Kit.
AutoLogin will not work if the External Graphics Development Kit is attached at the time the system is booted or restarted.
It can help to move the menu bar to the display connected to the External Graphics Development Kit. See System Preferences -> Displays -> Arrangement.