I'm pretty certain this wasn't happening with Mountain Lion I eject the external drives as one should through Finder, and then I unplug the USB and whichever HHDs were connected audibly keep running. I should mention that I use a USB hub. So I've up to three drives connected to the hub, and then just one USB connected directly into my MacBook. Now in order to get the drives to stop spinning after I've disconnected from my Mac, I need to disconnect them from the USB hub. Like I said, I really don't think this was happening before. I always kept my drives plugged into the hub and I wouldn't hear them going while not connected to the MacBook. Is anyone else experiencing this? Hopefully just something that's fixed in a near-future OS update?