This question already has an answer here:
I'm trying to upgrade from 12.04 to 14.04 and am encountering problems.
I'm working from 12.04. Downloaded the 14.04 iso file. Used the Startup Disc Creator App.
However, when I try to boot from USB, I get a "Boot error" message.
I've tried booting with the USB drive in all 4 USB ports. I've reformatted the USB drive. I've re-downloaded the iso file. Keep getting the same results.
Frustrating because it worked fine when I loaded 12.04 using this same procedure on the same computer (Toshiba Satellite with AMD 64 processor).