Fusion Drive constantly spinning up and down 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.
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.