I could be wrong but:
I am not aware of any external hard drive case you can buy that will power
down according to the power management settings in the OS.
I have seen this advertised as a feature of some newer external hard drive
packages, e.g. Western Digital, but the box says the feature is only
available via USB2 or Firewire connections and not ESATA.
Otherwise you must relieve the pressure on your hindquarters and physically
flip a switch to turn the darn thing off: unbelievable, huh?