Please follow https://support.apple.com/en-us/HT202796 link to check the “How to set up and use an external Mac startup disk”. ARC-8050T2 uses Option ROM firmware. To enhance system security, Mac computers with up-to-date software don’t automatically load Option ROM firmware. As a result, your Mac doesn't see devices that have Option ROM firmware until you load the firmware by pressing “Option-Shift-Command-Period” at the Startup Manager window. Do this each time you want to start up from the ARC-8050T2
If you're using an Early 2015 (or older) Mac model, you can disable this security feature and allow your Mac to load ARC-8050T2 Option ROM firmware automatically. This removes an important protection against potential unauthorized access to your Mac by other people with physical access to it.
1. Open the Terminal app, which is in the Utilities folder of your Applications folder.
2. Type sudo nvram enable-legacy-orom-behavior=1, then press Return.
To undo this command, enter sudo nvram -d enable-legacy-orom-behavior.
Tags: ARC-8050T2, firmware, Mac, Thunderbolt, Thunderbolt2