I recently sold my MacBook Air and shortly afterward the USB flash drive that I use to boot my FreeNAS server got corrupted leaving me in an awkward position of not having any easy way of recreating a new USB flash drive. I still, however, had my Raspberry Pi server which should have no problem writing a FreeNAS image to a USB flash drive. Here were the steps I used to get the job done.
Download the FreeNAS USB image
Identify the correct /dev path to your attached USB flash drive. There are a few different ways to do this
mount, lsusb, fdisk, dmesg, /var/messages or /dev/disk/by-id/usb-*
Update the /dev path to the USB flash drive in the following command and execute it (My USB flash drive is /dev/sdb)
sudo sh -c 'xzcat FreeNAS-188.8.131.52-RELEASE-x64.img.xz | dd of=/dev/sdb bs=64k'