iMac sometimes refuses to go to sleep

Thank you for this suggestion. The support document is helpful, but I did not see anything in it which I can determine is the culprit for this problem. What is puzzling is its sporadic nature and its seeming spontaneous emergence about a month ago. The only thing notable that preceded it was an incident where an audio CD would not mount properly or eject and the computer had to be restarted a couple of times. Perhaps I should do some disk/hardware checking.. Any other thoughts?

Thanks

OK, thank you, this is very helpful. What I am seeing is repeated messages like this:

9/25/12 5:39:25 PM kernel System Sleep

9/25/12 5:39:28 PM kernel Wake reason = EHC1

9/25/12 5:39:28 PM kernel System Wake

9/25/12 5:39:28 PM kernel Previous Sleep Cause: 5

9/25/12 5:39:28 PM kernel The USB device Keyboard Hub (Port 1 of Hub at 0x24000000) may have caused a wake by issuing a remote wakeup (2)

9/25/12 5:39:28 PM kernel The USB device Apple Keyboard (Port 2 of Hub at 0x24100000) may have caused a wake by issuing a remote wakeup (3)

Now, I don't understand this: my keyboard seems fine, and I'm not doing anything on it to cause a wake. However, my USB mouse is plugged into the keyboard, so maybe the mouse is causing the problem?

Do you have another interpretation of the messages?

Thanks

Thanks, I'll do these tests. I also have another mouse so I may try that too. I should mention one other thing that may be relevant: I have two USB cables plugged into the computer that are usually just dangling loose except when I use them to connect their respective devices: 1) for my iPhone and 2) for my camera. This has been the setup for a long time and I was told it should not cause any problems (I do it this way because it is a nuisance to have to fish around behind the computer screen to plug in USB cables there every time I want to connect phone or camera). I do frequently use the iPhone cable to charge/sync the phone, and I'm always careful to eject the phone before disconnecting.

Anyway, as I said, these cables have been there for many months and the waking problem is new.

Actually, there is a third cable dangling now, from the external HD which I unplugged a couple of days ago when I thought it was the source of the problem.

That is a lot of ports.

My experimentation so far has been to swap the mouse for another one, and so far I've put the computer to sleep a few times and it has stayed asleep. I'll want to test it further, but this is looking like it may be mouse-related. Perhaps one contributing factor had been wear and tear on the mouse including a few times I sort of slammed it down on the trackpad in frustration because the scroll ball was not behaving. Lesson: never vent anger with violence on your computer hardware.

Thank you for the cleaning tip. I'll try it. Have also had some success with a bit of iPhone screen cleaning solution on a tissue rolling the ball around.

So far the computer is not waking back up with the new mouse.

I wanted to update this discussion. A few days ago I swapped the mouse for a different one. The waking problem stopped. After a couple of days with that setup there were no waking problems, so I then swapped back the mouse to the original one with which I was having the problem (and which I supposed above I might have damaged). After a couple more days, there are no waking problems so far. The computer has been behaving fine for almost a week. Not what I was expecting. So at this point this is sort of an undiagnosed problem. It had been sporadic before, as I mentioned, but at the moment it seems to be in remission. So thanks for advice earlier. If things get confusing again, I may be back.

Thanks for the suggestion. In my case, this particular plugin is not part of the problem, but I had been considering the possibility that it was something software-related. Thing is, I could never find anthing (e.g. program running) that seemed to correlate with the problem. So in this case it's undiagnosed for the moment.