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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?

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3 Answers 3

Siracusa's review mentions this behavior, and it's been evolving since Snow Leopard:

DarkWake has actually been around since Snow Leopard, where it was used to periodically send out a network tickler to ensure that a sleeping Mac doesn't lose its DHCP lease. In Lion, plugging in or unplugging a USB device could briefly put a Mac into DarkWake to reconfigure the peripheral bus before going back to sleep. Mountain Lion does all of the above while also carving out some time to get real work done while the master is away.

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.

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I'm pretty sure this is intended, mine has the same behavior when connecting an iPod. I think this may have been changed on purpose. I know for myself every time I connected my iPod it bothered me that it woke my Mac up since I really just wanted to charge the device.

So I doubt it's a bug but rather intentional.

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Waking only for a short period after connecting or disconnecting a USB device is deliberate. Mac OS X 10.8, Mountain Lion, wakes long enough to set up or remove the device before returning to sleep.

Your Mac will only wake when user activity is detected or when previously scheduled to wake.

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