Hi there,
So I created a bootable usb to install Sierra. I used diskmakerx and the process was esty and fast. But when I want to boot from the USB it won't show up on the startup disk manager. I tried choosing it from system preferences and from the startup disk chooser by holding the option key when restarting. The drive does not appear on any of them, but it's perfectly vissible when the OS loads.
Of course I tried different USB sticks and no result.
Any ideas?