The mysteries of sleep: what’s making my hard drive sleep?

There’s something odd about this iMac17,1 (that’s an iMac Retina 5K 27-inch, Late 2015): it cannot put the system to sleep without putting its internal hard drive to sleep. It has been like this since new, through El Capitan and now Sierra.

When I enable system sleep in its Energy Saver settings, no matter whether the box below to Put hard disks to sleep when possible is unchecked or not, when it puts the computer to sleep it always sleeps the hard disk.


Even old and primitive Macs used to have separate timer controls for the display, the system or computer, and hard disks. Way back in Classic Mac OS, you could have separate periods set for each, and those ancient systems respected them.


With today’s wonderful advanced operating system, powerful integrated chipsets, and custom SMCs, you might have thought that such controls would have improved. But the only slider control that I get governs display sleep. I can also enable system sleep, and the pane pretends that I can determine whether or not hard disks are put to sleep too. I have no slider(s) to control the onset of system or disk sleep.

I have checked that this is not just a preference file problem: my .plist accurately reflects the settings in my Energy Saver pane, but those are just ignored by my Mac, which does what it wants to instead. I have also reset the SMC and NVRAM: it makes not a blind bit of difference.

It strikes me that there are several possibilities:

  • this is a ‘feature’ of Late 2015 iMacs,
  • this is a limitation of the SMC in the iMac17,1 model,
  • this is a limitation of having an Apple Fusion Drive,
  • this is just a bug.

Can you put your Mac into computer or system sleep without putting its hard disk to sleep? I’d love to hear of any information which might help clarify this. Meanwhile, it’s back on the macOS Sierra 10.12.5 bug list.