a very tired hard drive that does not recognize the wakeup call.
I would seriously consider making sure my Time Machine backup was up to date and consider replacing that drive. Although drives are cheap,(ish) there is a fair amount of working involved in getting it into the iMac.
Not as expensive as a logic board replacement though
Just after my warranty expired I developed what I suspect will be an expensive problem. Upon awakening from sleep my computer cannot recognize the internal HD. I total freeze. If I power down & reboot everything works.
If I boot the computer from an external HD clone of my internal HD a can put the computer to sleep and wake up without a problem EXCEPT:When I do this the computer removes the internal HD icon from the desktop and Iget a dialog box informing me that the internal HD was improperly disconnected.
Any ideas where the problem may lie. I fear it may be the logic board.