Fabian,
is there a specific reason why you want to boot the ISO from USB ?
The simplest and most straight forward approach to get a USB stick booting up
Trisquel 5.5 is simply to install it. You can treat the USB stick similar to
a small HDD. I run Trisquel 5.5 from a quality 16GB stick myself without
issues. I simply would do a custom layout and disable swap completely. There
are a few other tricks when running Linux from any flash-based media but most
of them are not mandatory anymore.
HTH,
Holger
