The same thing happened to me when I created a !trisquel 8 USB bootable drive. It immediately showed the grub for my !parabola machine. When I tried the same USB on a machine with no OS it said something along the lines of "No Boot disk detected". This started from the December to the latest ISO image. I'll wait for the final version before trying again. For the meantime I installed Trisquel 7 on my child's laptop.