My iMac likes to spin its HDD to impede sleep

[phos07]

Hi there!

I have an iMac that likes to spin its HDD in the middle of the night. It never bothered me until this week, and I would like to resolve this to guarantee myself a good night's sleep. I can hear the HDD start to spin after the computer goes into sleep numerous times, and it has become rather annoying.

I can provide any new details that are relevant, and I'll try to do so asap.

iMac (Mid-2011, 21.5 in)
OS: High Sierra
Internet: Ethernet
Peripherals: Bluetooth Apple Magic Keyboard, Wired USB Mouse, Lightning Cable (not connected to phone at night)
Issue: Ongoing for years. The HDD starts to spin after iMac goes to sleep. The HDD spins for around 30 sec ~ 1 min before stopping. The HDD turns on in short intervals (5 min between intervals), and sometimes even shorter. All while the display stays turned off.

 

[DLary]

Have you disabled power nap in Energy Saver settings? Wake for network access?

 

[IowaLynn]

Replace it with SSD.

Reset SMC/PRAM/NVRAM. When last did you do clean install?

 

[phos07]

Have you disabled power nap in Energy Saver settings? Wake for network access?

Apparently power nap isn't compatible with my version. Wake for network access was ticked so I turned it off. I though that was unchecked but I might have confused it with another iMac. I'll see if that solved it.

Replace it with SSD.

Reset SMC/PRAM/NVRAM. When last did you do clean install?

My last clean install was this Spring. I tried a SMC reset, but that didn't solve the issue. SSD replacement isn't an option for me.

 

[Ruggy]

A couple of things to check.
The first thing is to check is in the power saving preferences. If you set it to put the HD to sleep whenever possible that might help.
I don't think it's that from your description because that would tend to happen almost straight away but worth a mention.

There's also a setting to wake the computer on network access. This tends not to be so regular but this does have a habit of waking the computer from time to time in the middle of the night. So you can switch that off.

I also had this problem for a couple of years with an iMac and wired keyboard and mouse and eventually I found it was because I had the mouse set up to wake the computer when you move it.
I had the mouse on a computer drawer/tray and the slightest vibration would be enough to set it off and wake the machine. It would wake up then go back to sleep again several times a night.
Could that be it?
Best