Hard drive spinning up during sleep

[JonD25]

I'm running a 2014 iMac 5k and first noticed this issue with my external USB hard drives plugged in (2 Western Digital My Book drives). When I set the Mac to sleep at night, the external hard drives spin down as well, until a second later when they spin right back up again. I started just unmounting the hard drives and unplugging, inconvenient but not a total pain. But I noticed that I still hear my internal drive spin up as well (1TB Fusion drive). It's usually at somewhat random intervals for only a few minutes at a time. I've disabled "Power Nap", "Wake for network access", and enabled "Put hard disks to sleep when possible" and still have the issue. I've also added the external drives to be ignored by Spotlight. Not sure what else could be going on. Any ideas?

 

[Thomi2]

If you notice hard drive activity, even during sleep, it necessarily mean your drive is activated.
Actually, it's no worry that your hard drive request access/perform tasks sometimes, but it should not happen when sleep especially if you told OS X to pause disks activity.
Your internal hard drive may be processing some tasks, check the memory usage, or monitor the processes used and currently in use within OS X with the stock app provided.

But if you say your drive still works when sleep mode, looking for a constant running process might not be the problem solver.
Hope that helps.

 

[JonD25]

I'm not really sure I understand what you're suggesting? How would I be able to monitor activity when the computer is asleep? When the external hard drives are attached, I can hear them spin up, and even when they aren't I can hear the internal spin up occasionally too. If I wake the computer to check the activity, wouldn't the processes that start when the computer is awake pop up and not just what was active when asleep?

 

[JonD25]

I've tried logging out and into an empty new account, still happens. I've tried logging out entirely, then sleeping, still happens. It seems whatever is keeping the hard drives active is a system level thing if it happens even when I'm logged out, right? Last night, with the external drives unplugged, I heard the internal drive spin up and down several times. If I leave it like that overnight, that sounds like a considerable amount of wear and tear over time. Can anyone help figure this out???