Some users have reported issues after applying Security Update 2010-05, the most dire of which is an inability to startup (boot) directly after updating. If you experience this issue, or another show-stopping problem, follow these potential fixes:
Delete errant kernel extensions. Early indications are that this issue could be related to a kernel extension (.kext) or kernel extension cache file. Kernel extensions are add-ons to the Mac OS X kernel, often designed to facilitate interaction with third-party devices or software. Specifically, some users have reported that deleting the following kernel extension:
then restarting may resolve the issue. PACE is part of an anti-piracy package installed by some software vendors.
Clear kernel caches. Some users have also reported at least temporary respite from this issue via clearing of carious caches (including kernel extension caches) with a tool such as AppleJack. To follow this routine, download AppleJack and install it, then restart your Mac in single-user mode by holding down the command and S keys at startup. At the prompt, type “applejack auto restart” (with no quotation marks) and press return [note that while AppleJack 1.5 is technically incompatible with Snow Leopard, only the "repair permissions" function fails, which is not necessary for this process. If you'd like, you can use an alternative cache clearing tool such as YASU.]
Reset PRAM/NVRAM. This fix has worked for previous instances of this issue. See this Apple Knowledge Base article for instructions for performing this procedure.
Remove third-party startup items. Third-party startup items may cause this issue. Look in the folder /Library/StartupItems/ for any third-party startup items and temporarily remove them then restart.’
Disconnect USB devices. Incompatible or otherwise problematic USB devices can cause startup problems. Try temporarily disconnecting any peripherals then re-attempt startup.
Failing the above, try the following:
Option #1 — try this first
Download the Mac OS X 10.6.4 combo updater, which weighs in at a hefty 887MB but is a troubleshooting godsend. Quit all open applications and run the installer.
Option #2 — if option #1 fails, try this.
Booting your Mac in Safe Mode then simply restarting normally is one of the most overlooked, most effective procedures for solving a variety of issues after an incremental Mac OS X update. The reason for its efficacy: booting in Safe Mode forces a disk directory check, clears potentially problematic cache files and performs other routines, detailed here.
To boot in Safe Mode, hold the Shift key while your Mac is starting up. After booting in Safe Mode, simply restart normally (without holding the Shift key) and check for persistence of the issue.
Option #3 — if options #1 and #2 fail, try this.
Remove the update. Although this option eliminates important security fixes included in the update, it may be the best option if a problem is interrupting your workflow and options #1 and #2 prove ineffective.
Reinstall (if you don’t have a current Time Machine backup). Insert your Snow Leopard installation disc, then restart while holding down the C key. When prompted, choose the normal “Install” option. Make sure to select “Preserve Users and Network Settings.”
After installation, you’ll be left with an earlier iteration of Mac OS X 10.6.x (most current retail discs include Mac OS X 10.6.2), but an otherwise largely intact system. Download the Mac OS X 10.6.4 combo updater and apply it if your disc has an earlier version of Snow Leopard. You may need to restore some saved username/password information, browser information, etc.
Restore from Time Machine Backup. If you have a current Time Machine backup: connect the Time Machine backup disk, then insert your Snow Leopard installation disc, then restart while holding down the C key. After selecting your language, go to the Utilities menu and choose”Restore from Time Machine Backup.” Choose your Time Machine backup disk and choose a backup iteration directly prior to when you installed the Security Update.