https://support.apple.com/en-us/HT202796 \\ 
option+command+shift+period at boot up manager to load option ROM firmware \\ 

If you can't select your startup disk or start up from it

Some third-party external devices use 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 device or from a startup disk connected to it.

If you're using an Early 2015 (or older) Mac model, you can disable this security feature and allow your Mac to load Option ROM firmware automatically. This removes an important protection against potential unauthorized access to your Mac by other people with physical access to it.

Open the Terminal app, which is in the Utilities folder of your Applications folder. 
Type <code>sudo nvram enable-legacy-orom-behavior=1</code>, then press Return.
To undo this command, enter <code>sudo nvram -d enable-legacy-orom-behavior.
</code>