Please update the firmware package to V1.54 2017-08-19 or later and follow the https://support.apple.com/en-us/HT202796 link “How to set up and use an external Mac startup disk”. ARC-8050T3 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-8050T3. Apple macOS 10.12.3 and 10.12.4 have issue to load the devices Option ROM firmware .This issue has been fixed on the macOS 10.12.5 or later version.
Before start multi-boot, you need to prepare another Mac OS disk on 8050T3. Use backup software like "Carbon Copy Cloner"(30-day Trial) to clone operating system on 8050T3 (the destination disk needs to format). When you have prepared the OS on external storage device, startup disk app will show an example like below:
However, Mac computer with up-to-date system doesn’t automatically load third-party external devices with Option ROM firmware, so you have to use Startup Manager into 8050T3's OS.
*Make sure BIOS Selection as UEFI on the System Controls: Advance Configuration. 1. Hold down the Option key immediately after turning on or restarting your Mac. The screen will show all the startup system on internal hard drive.
2. Release the Option key when you see the Startup Manager window.
3. When 8050T3 is ready, pressing Option-Shift-Command-Period at the Startup Manager window.
And both internal and external startup disk will show in the screen.
4. Choice the disk you want to start up.
5. After into the system, you can check "About this Mac" to make sure is right startup disk.
Tags: ARC-8050T3, firmware, Mac, Thunderbolt3