Not able to use the dd command to put an img file on USB stick
Try this
Open a terminal,
Press Ctrl+Alt+T
Run it:
exec sudo -i
fdisk -l
Fdisk will tell you how your usb really identifies itself. Suppose it is /dev/sdc, continue running:
dd if=/home/jan/Downloads/COS/chromiumos_image.img of=/dev/sdc bs=4MB
Change /dev/sdc to the one corresponding to your usb