OS X System Upgrade - progress bar stuck - not making progress

Details

Selected to do an OS X system update (such as a security update) and the update starts, but the progress bar never completes.

If the user has waited a significant period of time (like an hour or more)...

Options in this scenario can be:

  1. Hold down the power button to shut down the computer, then restart.
    • Sometimes the update never really got started, so reboot is normal.
  2. If the restart results in the computer failing to complete startup process, then
    • Restart into Recovery mode (Command-R), then reinstall OS X.
    • Data and Applications are preserved.