I succesfully created an Ubuntu 18.04.3 LTS bootable USB stick following this tutorial.[1] I used it with two different Apple Mac computers, and on both occasions, after selecting on startup either of the two gold/yellow ‘EFI Boot’ options (I'm presented with two), before reaching the GRUB menu I see the following: "Error: file /boot/ not found" (or something along those lines). When I reach the GRUB menu, I select to run Ubuntu without installing, and I get a black screen, after which I have to force shutdown the computer.

[1] https://ubuntu.com/tutorials/tutoria...stick-on-macos