Regardless of how you have decided to install OS X (Software Update, Delta, or Combo installers), you may run into a problem where the system might hang on installation. Though the occurrence of this with the 10.6.3 update seems to be rare, when it happens you may see it sit forever with the spinning color wheel, or go blank to a blue, black, or gray screen and require a forced restart. Even if the system appears to be healthy after the forced restart, there is the possibility that some files could be corrupt. As a precaution we recommend reapplying the combo updater.
Most hangs at installation usually happen when the system is running more maintenance-based routines or performing commands like restarting, so the system will not necessarily be hurt; however, there is always the chance that files can get corrupted by interrupting the installation process.
If your computer hangs during installation, try the following steps:
Give it time
Many times the system will sit at a blue or black screen for a while, so be sure to give it ample time before concluding the system is hung up. Let it sit there for about half an hour to see if it resolves the hang and continues the installation process properly.
Hard-reset only if the system is not doing anything
If the system does not respond after waiting, only hard-reset it if the hard drive is not working. Put your ear to the case of the system to hear if the drive is working. If so, wait until you cannot hear the drive chattering away and then press and hold the power button until the system shuts off.
Immediately boot to safe mode
Once the system has been powered down, boot it up and immediately press the Shift key to go to Safe Mode. This will run some diagnostics scripts at boot-up, and also load the OS in a minimal way to prevent any interference. When booted, run Disk Utility's permissions fix and hard-drive verification routines, as well as run any maintenance utilities you may have for cleaning the system's temporary files (caches, etc.).
Reapply the combo updater
Even if the system seems to be working fine, after any fault in the installation it is always best to re-run the installation using the "Combo" updater. This will ensure that all installed files are in working order, and prevent any currently unused but corrupt files from causing problems later on. We recommend you download the Combo updater and run it when booted in Safe Mode.
Check permissions after installation
Once you have completed the installation with the Combo updater, use Disk Utility to run a full permissions fix on the hard drive. This will ensure that all updated files are properly accessible by the system, and prevent slowdowns and hangs that could result if the system cannot access these files.
Start over from backup
If the installation is still not working properly, even after reapplying the Combo updater, then it is highly recommended you start over. This can be done by reverting to a backup you made before applying the 10.6.3 update (Time Machine or a cloned drive), or by performing a reinstall of the OS from the Snow Leopard DVD. When you have reverted to the backup, be sure to fully prepare your system for the update by following the procedures suggested in.
Reverting to a backup is the preferred method, since it will keep all of your settings and program installations intact; however, some people may not have this in which case a reinstallation may be necessary. Snow Leopard will perform an archive and install, which will preserve user data and installed applications, so reinstalling should keep most settings intact.