I had a Ubuntu live USB that worked fine on a MacBook Air. I tried to boot it on a Mac Pro, and it seemed to get stuck on [sdb] Assuming drive cache: write through, but the real first error was the one that needed fixing, and it was i8042: No controller found.

I finally found the fix on Ask Ubuntu, which is to—when the live USB boots up—press e to edit the Grub line, then replace quiet splash with

Press F10 to boot from the edited line, and it should work just fine.