IMac 2007 model hangs after waking from sleep

Question

Hi there,
I have a 2007 iMac with 2Ghz processor, 4GB RAM running OSX 10.8.2 which has been a fantastic computer up until a few weeks ago. I hardly ever turn my computer off so it falls asleep during the nights. When I wake it though it seems to hang. It doesn't really matter what I try and do it just sits there for ages thinking about what it's going to do. Sometimes for ten minutes before an app will finally open. Once it has settled and decides its going to work its back to its usual self and working fine.
I have tried formatting the hard drive and reinstalling everything, I have also reset the PRAM, and checked in the disk utility that the SMART setting says verified on my HD. I have run software updates so everything is bang up to date, but I'm still having issues. If I reboot and start a fresh it seems to work, but doing this every time it falls asleep seems wrong and unnecessary, plus whilst its hanging it won't let me reboot, I have to manually turn the power off which I don't like doing.
Has anyone got any ideas as to why this is happening? I have not upgraded anything hardware wise, apart from the RAM but that was years ago, so has no impact. Hope I've put enough details here, and thanks in advance for any ideas
Sam

Answer 1

I believe that this is an artifact of waking up from "hibernation" or "deep sleep"
there are several sleep modes:
regular sleep just cut off power to disk and monitor, but keeps the RAM powered.
safe sleep keeps RAM powered but also writes the contents of RAM to disk, just in case power is cut off (usually for laptops where battery can be drained/removed), but normally would wake up from RAM
deep sleep is full on hibernation, RAM is written to disk and the machine is shut down.
check your hibernatemode:
pmset -g | grep hibernatemode
0 = no hibernation i.e. regular sleep
1 = immediate hibernation i.e. deep sleep
3 = safe sleep
25 = agresive pageout with deep sleep.
therefore it could be that the "sitting and thinking" is your mac reading back the RAM from the hard disk after waking up from deep sleep...
check your hibernatemode.
my first iMac (27" late 2012) exhibited the fan going on full blast upon connecting to power, quiet when the machine was booted, and full blast again when the machine was off.
I am now on my third replacement iMac, the last two are exhibiting identical issue with sleep: the machines will go to sleep and will not wake up, and in addition the power button is disabled, i.e. pressing the power button will not restart the machine.
I am guessing that the SMC crashed, as the only course of recovery is to reset the SMC by removing power, pressing the power button for a while, and attempting to restart (does not work all the time, so I sometimes have to repeat the SMC reset procedure).
after doing some research on the net, I found how to view the hibernate mode, and saw that my machine was set to hibernate mode 5, which is NOT an approved setting, see below.
I used the system pref -> energy saver and clicked restore defaults which restored the hibernatemode to 0.
HOWVER, upon plugging in a powered USB hub, the hibernate mode was automagically reset to 5.
once at hibernate mode 5, putting the machine to sleep manually exhibited the same symptoms, i.e. not wake from sleep and disabled power button.
this was done with an apple care tech on the phone, and he recommended replacing the iMac once again, and sending me a fourth replacement!
overnight I played some more with this
I reset the hibernate mode but this time to 3 and I tried a different USB hub.
hibernate mode stayed at 3 after plugging in the USB hub.
I then created a small shell script to monitor and display the hibernate mode:
function showhibernate() {
  while true
    do echo -n `date` " ";pmset -g|grep hibernatemode;sleep 10
  done
and executed it overnight
this evening after I came home from work, I noticed that the  hibernate mode somehow was switched from 3 to 7!?
unfortunately, I did not have the echo `date` on the first version, so I do not know when it was switched from 3 to 7.
however, the machine did wake up from sleep... (so it seems that 7 is not as bad as 5?)
I am guessing that some powered USB hubs cause unexpected changes to the hibernatemode, which in turn cause the SMC to crash, disabling the power button.
why plugging in some powered USB hubs but not others will cause the hibernatemode to switch to 5?
why hibernate mode would automagically switch from 3 to 7?
neither 5 nor 7 are documented in the man page, and the only recommended modes are 0 3 and 25:
We do not recommend modifying hibernation settings. Any changes you make are not supported. If you choose to do so anyway, we recommend using one of these three settings. For your sake and mine, please don't use anything other 0, 3, or 25.
For now I cancled the shipment of the fourth replacement, as this seems to be an issue affecting all iMacs (and a specific USB powered hub), so that a new machine will not resolve the issue...
bottom line, remove the USB hub, and/or try another one.
hope this helps.