I am looking for an external HDD that will spin down and go to sleep when the iMac does. I currently have a USB powered one that does not do this, and keeps me awake. I want to use it for Time machine, so it'll need to be a 3.5inch and therefore mains powered unit. Can you get drives that do this, or does their power just stay on until you flip their respective power switches? I have heard that buffalo drives will dim their lights when they sleep, does this mean they will sleep when the iMac does?