Hlipka has written an article about his auto-power-off project:
I have an external hard drive connected to our satellite receiver for quite some time (to have it work as a PVR). But for some reason the disk won’t power down even with the receiver switched off (I verified – the disk itself is configured properly). So yesterday I went ahead and added small circuit to the enclosure switching it off when the USB master (the receiver) is turned off. The circuit I used looks like this.
Via the forum.