On Snow Leopard (10.6), I could wake my Mac by connecting a USB device – in my case, switching on a USB external HDD.
Now, after upgrading to Mountain Lion (10.8), when I connect the USB device, the Mac wakes up and then immediately goes back to sleep.
Is this a bug? Is there a reason for this new behaviour?
Best Answer
Siracusa's review mentions this behavior, and it's been evolving since Snow Leopard:
DarkWake is the state the system is in when Power Nap is running. In Lion, it would enter DarkWake to deal with processing the USB connection, then it would go back to sleep. In Mountain Lion, when a USB device is connected, the Mac enters DarkWake to deal with processing the USB connection and also uses this state to do the various Power Nap tasks.