Some leds, on some boxes models, are kernel controlled and can't be changed, others are hardware controlled, and still others are Alt-F controlled, so if you mess with them unpredictable led behaviour might occur.
That said,
echo 1 > /tmp/sys/left_led/brightness # will turn left orange hdd led on
echo 0 > /tmp/sys/left_led/brightness # will turn left orange hdd led off
echo "timer" > /tmp/sys/left_led/trigger # will make it blink
echo "none" > /tmp/sys/left_led/trigger # return to normal