Might be a faulty USB... I switched to another USB to install from and tried to install to a HDD instead. This time it worked better and i was able to install.
I have since also tried to install it again, (since I want it installed on a USB key instead) but I can't install to the Patriot USB stick (16G) it seems... The installation either hangs or it's perhaps very slow. Is there any new special requirements for the USB that you install FreeNAS onto?