How to determine when a Mac is in Power Nap state? 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?
There is the simple answer and the clever answer.

The simple answer is listen for the external drive spinning. If it's sleeping, you will not be interrupting data transfer (but you will be yanking an open filesystem from a sleeping computer but that's not a new issue introduced by Power Nap).

The more clever answer is to observe other things that may be happening like network traffic by sniffing the traffic and seeing if the machine is sharing files or available to ssh into if you have sharing turned on. This takes more work and an iOS device or another computer and certainly won't be faster than just opening the lid of the Mac and knowing it's not napping anymore.

In the end - it's probably worth re-training your workflow to just open the lid and eject things properly. Since Power Nap is only supported on Mac that have Apple supplied integral SSD and ridiculously fast wake times, the delay to perform the eject cleanly is less than 10 seconds excluding however long your external drive takes to wake.