Grub2: boot USB from GRUB prompt
When the PC will not boot from USB, you can use GRUB2 to boot from the USB.
In the GRUB menu: Press 'c' to go the GRUB prompt
grub> ls (hd0) (hd0,msdos1) ... grub> set root=hd0,msdos1 grub> chainloader /efi/boot/bootx64.efi grub> boot
The USB boot menu will appear.
The above is of course an example, every situation will be different. But the idea is the same.