Enabling external booting on a fusion drive Mac
Option 1)
Wait for Internet recovery to finish loading, proceed with normal steps.
P.S. this shouldn't be necessary unless you've enabled the option to disallow booting from external media. If that is the case, see this article for instructions: https://support.apple.com/en-us/HT208198#open
Option 2)
If you're impatient like me, go to System Preferences &gt; Startup Disk.
Click on the drive you wish to boot from, click Restart.