Comment 10 for bug 315970

Thanks for reporting this bug and helping make Ubuntu better. The bug you have mentioned has been debated by the developers, and they have decide to not fix the bug. There reasoning is explained below, and can be found at:
http://live.gnome.org/GnomePowerManager/FAQ#head-e6d48e4196eb363908fb71975a5311f11f8dd9e4
Please report any additional bugs that you find!

GNOME Power Manager doesn't spin down my hard-drive!

After numerous debates, the consensus was that is was not a good idea to add this functionality to HAL. It's was decided user-configurable powermanagement was not really required when modern hard disks have really intelligent powermanagment.

A disk on Low Power idle need less than 1 Watt per hour. For a normal battery with 50000mWh you could run the harddisk for over 50 hours. If you do not read/write from/to the harddisk the disk regulates power, but never shuts down the device. The reason is easy: you lost more power with each startup than to leave the harddisk online somewhere between 'Active idle' and 'Low power idle' (depends on the model/manufacturer). The other reason to leave this to the internal powermanagement of the disk is: the time needed to reactivate the device. You lose more performance than you lose power between 'Active idle' and 'Low power idle'. If you use a journaling file system you normally need to flush periodically. This could run in a race between shut down device and restart device by system to flush. This means more power consumption as you change nothing. You can't set powermanagement for exteral USB harddisks, because you can't send the needed commands over the USB link to the disk.