Recent hard disks by Western Digital don't support APM and implement their own policy for power saving. This can be annoying - or even fatal - for some disks, e.g. the head is parked after only 8 secs of inactivity.
I've seen that a first (commented out) hack has been attempted to support the idle3 command(s) (forgive my possible lack of precision); I didn't dare try it before it's deemed stable enough.
I'm the (not so happy) owner of 12 WD Caviar Green 2TB, and I'm humbly willing to assist if it can help speeding up the implementation of the feature.