Many BIOSes prevent the user from booting from a USB disk at least if the MBR
is used. But there might be a bug in the trisquel installer image.
EFI boot is often buggy, as vendors only test with Windows. Installing an
GNU/Linux os in CSM mode might still detect an efi runtime and change EFI
settings.
You could try to install Ubuntu first, and then upgrade Ubuntu to Trisquel.
I think both versions of Ubuntu are OK for this porpuse.
http://trisquel.info/en/wiki/migrate-ubuntu-trisquel-without-reinstalling
