u could use a photo diode and read it via an analog pin...
or u read the LED power line directly...
http://arduino.cc/en/Reference/AnalogRead
instead of reading the HDD LED:
u could send the arduino a message every 5 seconds...
and when the arduino misses that message for 20 seconds, it "presses" the reset button...
in order to "press" the reset button, u could use a relay...
if the arduino and the supervised box use the same power supply: u connect the reset-signal line directly to the arduino and pull it down (IIRC)...