Why does my iMac not shutdown?

Question

Occasionally my imac will not shutdown.  I have a 2011 Core i5 iMac.  When I go to shut it down it will get to the screen with the spinning gear thingy and it will not shut down.  After the screen goes to sleep it looks like the computer has shut down, but with the click of a mouse you find that it is still trying to shutdown.  This doesn't happen every time I shutdown, but it happens enough to be annoying.  Any ideas on how to fix my problem?

Answer 1

Then, I'd suggest you reinstall Lion on a freshly formatted drive then reinstall your third-party applications being careful that they are all known to be compatible with Lion. You may want to backup your files:
Install or Reinstall Lion from Scratch
If possible backup your files to an external drive or second internal drive.
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Erase the hard drive:
Select Disk Utility from the main menu and click on the Continue button.
After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Note the SMART status of the drive in DU's status area.  If it does not say "Verified" then the drive is failing or has failed and will need replacing.  SMART info will not be reported  on external drives. Otherwise, click on the Erase tab in the DU main window.
Set the format type to Mac OS Extended (Journaled.) Click on the Erase button and wait until the process has completed.
Quit DU and return to the main menu.
Reinstall Lion: Select Reinstall Lion and click on the Install button.
Note: You can also re-download the Lion installer by opening the App Store application. Hold down the OPTION key and click on the Purchases icon in the toolbar. You should now see an active Install button to the right of your Lion purchase entry. There are situations in which this will not work. For example, if you are already booted into the Lion you originally purchased with your Apple ID or if an instance of the Lion installer is located anywhere on your computer.