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I've got a 1TB Fusion Drive (with a Seagate 7200 rpm hard drive) in a Late 2013 27" iMac. While the iMac is asleep, the hard drive spins up and down every minute or so (without fail), creating an annoying rattling sound as it spins up - characteristic of hard drives. This is incredibly annoying, especially at night.

It doesn't exhibit this behaviour when it is powered on, just when it is asleep. I've tried disabling Power Nap, and checking and unchecking "Automatically put hard disks to sleep when possible", all to no avail.

It sounds more like a software issue than a hardware issue, has anyone else experienced similar behaviour with the Fusion Drive?

Is there anything I can do to stop this? It's driving me crazy.

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There are 2 ways to look what is it doing in sleep.

One is open the Console in utility, then look at the sleep hours log messages.

If you need help with that publish a copy of that segment here in your question.

The second way is using the Terminal in the utility.

In Terminal window type pmset -g

That will show you a list of sleep mode settings and who is preventing the sleep.

  • After running pmset -g, I found out that I had left Internet Sharing on. Thanks for that. – Jack Greenhill May 10 '14 at 4:37
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I had this exact problem and it was driving me nuts! The iMac was going to sleep, I could hear the HDD stopping, then a few seconds later it would spin up for about 20 seconds, stop, wake up again a few seconds later... and so on.

Tried checking print queues, even removed old printing software as a precaution, turned off power/wake settings, checked diagnostics, logged in as a different user, tried safe mode... no good.

Then I thought about what most recently happened around the same time. Other that upgrading to El Capitan, my wife needed a new laser mouse urgently. I bought a plain black cheap-and-nasty $5 J.BURROWS brand "Made in China" model from Officeworks to get by. Great mouse and great value, but there is something quirky about it that keeps triggering the HDD to wake up.

Cheap mouse gone, no more spin-up issues!!!

So, if you're experiencing this problem, before you dive into technical software tweaks, go back to basics and try changing keyboard, mouse or any new peripheral you've recently added.

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