I have two external HDs I can use via USB to do TM backups on my new late 2013 iMac. If, however, I have any of the bottom four settings invoked on the iMac's Energy Saver app, the power to the USB bus on the iMac also shuts down, I get an error message that the external drives were improperly ejected and, of course, the TM backups are not down. The external drives are hooked up via their own power supplies to AC, and the drives are plugged into the back of the iMac. One of the drives is a Lacie, and the other is from OWC. Their operation is perfectly normal otherwise. I've attached a snap of the Energy Saver app. Is this a known issue...the loss of connection between external USB drives when the iMac goes into any of its power-saving modes?