HT5964: About the security content of the OS X Mountain Lion v10.8.5 Supplemental UpdateLearn about About the security content of the OS X Mountain Lion v10.8.5 Supplemental Update
Currently Being ModeratedOct 12, 2013 9:00 PM (in response to Jarsonic)
Press and hold the power button for a few seconds until the power shuts off, then release, wait a few more seconds, and press it again briefly to restart.
Currently Being ModeratedOct 12, 2013 9:04 PM (in response to Jarsonic)
If it's still not done then I'd see if you can quit the installer (force quit is command-option-escape).
Next I'd make sure that there is a backup.
Then I'd boot into the recovery partition and repair your hard drive (repair permissions while you're at it.)
Then try the Update again.
Or you might want to wait a few days for Mavericks to come out.
You might fare better with the Combo Updater. OS X Mountain Lion Update v10.8.5 (Combo)
Currently Being ModeratedOct 13, 2013 4:49 PM (in response to Jarsonic)
Thanks, I was concern about the well known phrase "do not turn off your computer until the installation finishes".