My 2008 iMac/10.10.5 has recently refused to sleep... either automatically or with a menu command.

The internal HDD failed some time ago, since which it's been running fine on an external USB har drive.

No 'sharing' options are enabled. 'Wake for network access' is not enabled. 'Allow Bluetooth devices to wake...' is not enabled. The issue also happens with a 'test' account.

SMC and NVRAM have been reset.

pmset sleepnow, osascript -e 'tell application "System Events" to sleep' and osascript -e 'tell application "Finder" to sleep' immediately sleep the display but not the system.

The Energy Pane of Activity Monitor doesn't show any apps preventing sleep.

I'd had an issue with addressbooksourcesync preventing sleep, but think that's now sorted.

pmset -g assertions shows '0' for PreventUserIdleDisplaySleep and PreventUserIdleSystemSleep.

Assertion status system-wide:
   BackgroundTask                 0
   ApplePushServiceTask           0
   UserIsActive                   1
   PreventUserIdleDisplaySleep    0
   PreventSystemSleep             0
   ExternalMedia                  1
   PreventUserIdleSystemSleep     0
   NetworkClientActive            1
Listed by owning process:
   pid 195(cupsd): [0x0000004600110135] 00:10:00 NetworkClientActive named: "org.cups.cupsd"
   pid 49(powerd): [0x0000003c0008012c] 00:10:11 ExternalMedia named: "com.apple.powermanagement.externalmediamounted"
   pid 95(hidd): [0x000000a500090183] 00:08:25 UserIsActive named: "com.apple.iohideventsystem.queue.tickle"
    Timeout will fire in 218 secs Action=TimeoutActionRelease
Kernel Assertions: 0xc=USB,BT-HID
   id=500  level=255 0x4=USB mod=21/12/2016 12:46 description=EHC2 owner=AppleUSBEHCI
   id=501  level=255 0x4=USB mod=21/12/2016 12:46 description=EHC1 owner=AppleUSBEHCI
   id=503  level=255 0x4=USB mod=21/12/2016 12:52 description=UHC4 owner=AppleUSBUHCI
   id=506  level=255 0x4=USB mod=21/12/2016 12:55 description=UHC3 owner=AppleUSBUHCI
   id=507  level=255 0x8=BT-HID mod=01/01/1970 01:00 description=com.apple.driver.IOBluetoothHIDDriver owner=BNBMouseDevice

1 Answer 1


This is the key to the entire issue:

The internal HD failed some time ago, since which it's been running fine on an external USB hard drive.

It can't go to sleep when your USB is your boot drive. If it did put the port to sleep, it would unmount the boot device which would cause OS X to crash. You can confirm this with the line in your pmset output:

ExternalMedia 1

(The USB drive is, by definition, "external media")

If you replace your failed drive with a new internal drive or SSD, the problem will go away.

I had a failed drive in my iMac and I can understand why you have not yet changed out your drive. It's much easier than you think to replace it, it just takes some time. Even though ifixit rates it as "difficult", it's not difficult per se; it's time consuming and detail oriented. You have to do this very carefully.

Before you undertake this fix, make you you have all the parts you need like the replacement drive and the adhesive strips. Get quality ones and not generic double sided tape - I have seen people that went cheap here and had displays fall out - quality tape is much cheaper than a replacement display.

Have a look at ifixit.com for step-by-step instructions.

  • 1
    I don't know enough to pretend to understand this issue, particularly the 'ExternalMedia 1' status, but as it's been sleeping fine for many months 'usb as boot drive' is clearly not of itself the issue.
    – gulliver
    Commented Dec 21, 2016 at 17:18
  • This is disappointing if true, other machines / OSes do not suffer from such a restriction. Even a used or non-genuine SSD is several times more expensive than my PC laptop and not an option for me right now. Commented Aug 20, 2017 at 2:15
  • 1
    @hippietrail - something is keeping the drive mounted - it can be an app or a process (it could be more than one). The way to find it is to quit your apps one by one until you find it. But if you have to quit apps to put your machine to sleep, you're basically shutting the system down which defeats the purpose of putting things to sleep.
    – Allan
    Commented Aug 20, 2017 at 12:23
  • @Allan: Are you sure of this or is it just a theory? I think it might be by design, something to do with detecting USB devices be attached and removed though there could be a way to override that. If you know as a certainty then a page with more info about that or how to find in some log what is blocking it would be extremely useful. Commented Aug 21, 2017 at 0:21
  • Not so much a theory because as reported by pmset the external storage device is what is preventing your machine from sleeping. Some process has files open and this is what prevents it from hibernating/sleeping. You can try lsof to see what has files open and start from there, but it will ultimately be process of elimination. Thing is, if you have to close several/all your apps - you might as well shutdown.
    – Allan
    Commented Aug 21, 2017 at 0:56

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .