I've got a mid-2011 Macbook Air running Mountain Lion. Power Nap is enabled. My Time Machine back up goes to a USB external hard drive. My current workflow is to sleep my computer when not in use, and now that I'm running Mountain Lion I'll then attach the hard drive. Overnight, the system will use Power Nap to make a backup and all the other stuff Power Nap does.
My concern is that I don't know when it's safe to unplug said hard drive. The computer doesn't have any outward signs when it's in Power Nap vs. sleeping. So I'm afraid of unplugging the hard drive mid-backup. Having to wake up my laptop, eject the drive, and then re-sleep the laptop is something of a hassle, which I'd like to avoid if it's not necessary.
How can I determine when my laptop's in a Power Nap state instead of regular sleep?