After looking for a solution for a while I’ve found a way to powerdown external USB harddrive:
sudo sdparm –command=stop /dev/sdb
Full script for Jaunty:
sudo udevadm control –stop-exec-queue
sudo sdparm -6 -f –command=stop $1
read -p “Unplug the media and press any key to continue”
sudo udevadm control –start-exec-queue