fdisk -l
You'll see listing of drives available on your computer. Normally the USB drive will be shown like this :
/dev/sdb1 or
/dev/sdc1
Now run this command:
sudo umount /dev/sd<?><?>
where first question mark is letter and second question mark is number. Then run this command:
sudo dd bs=4M if=the_image_file.iso of=/dev/sd<?><?>
Wait until finish and run command
sync
Done!