The best way to avoid this problem is to understand how your BIOS is setup to boot.
Often newer machines will default to U(EFI) which is different than the traditional MBR/Legacy mode.
The problem is that this may not be apparent, often a BIOS Boot Menu will show a Legacy Boot Option and EFIOption without defining it.
A good example of this is if your USB is called "Kingston" you may see in your Boot Menu "Kingston" and also "Ubuntu"........