I know I've had some issues a few times making a usb live installation when I
was on other distros - sometimes unetbootin fails.
Just yesterday I downloaded the debian iso (stable 7.7) and tried to make a
usb image but didn't succeed with the startup disk creator tool. It gave me
an error when it reached 99%, that is when it has to make the bootloader..
Then I searched for unetbootin but I didn't find it in Trisquel. Then I moved
on my friend's laptop (xubuntu) and made one usb image with unetbootin but to
my great surprise that didn't work either - during the installation at some
point it failed..
I had to do it by terminal (never did that, but it is quite simple actually)
sudo dd if=/path-to-the-iso-file/filename.iso of=/dev/sdb