How to address a constant reboot loop in OS X-gctid49685

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How to address a constant reboot loop in OS X | MacFixIt - CNET Reviews

    by Topher Kessler    February 23, 2012 4:01 PM PST  
 
   If your Mac is stuck in a reboot loop, then it's likely suffering from corruption in kernel extensions or similar low-level files.

 

When you start up your Mac, the system will generally show a gray  Apple logo for a brief period followed by the log-in screen or other  user interface elements as the rest of the operating system loads. In  some instances, after crashes, power outages, or changes are made to the  system, instead of booting properly the system might display the Apple  logo and then reboot, repeating over and over.

 Once the OS X kernel loads properly, even if there is a problem with  background services, the computer will still boot but will just display  unwanted behavior such as location services or Spotlight not  functioning. Therefore, if your Mac is constantly rebooting after  displaying the Apple logo, then there might be a problem with the kernel  extensions that is preventing the system from booting properly.

 In the OS X boot process,  when the Apple logo is showing, the system has found a valid boot  device and will then display a spinning wheel below the Apple logo when  the system loads the kernel and its extensions. At this point, the  system can load kernel extensions from several sources including boot  caches and from the extensions' locations on disk. In doing so, odd  problems with the files from any of these sources could result in  problems that force the system to restart.

 To tackle a repeated reboot, the first step is to see if you can get  the system to load properly in any Mac operating system environment.  First try booting to Safe Mode by holding down a Shift key when the  system restarts. If the computer is able to boot properly, then you know  the issue is likely with some third-party extension or add-on you have  installed. This is especially true if the system only boots to Safe Mode  and continues its reboot loop when you restart without the Shift key  held down.

 If you cannot boot to Safe Mode, then try restarting to another OS X  installation such as that on an external hard drive or the OS X  installation disc (Lion users can boot to the Recovery HD partition by  holding Command-R at startup).

 If the system shows the same behavior when booting to a different  boot drive (especially to the OS X installation drive), then it suggests  a hardware fault is to blame for the problem, and not something to do  with a configuration of the operating system. In this case, you can try  resetting the PRAM and SMC for your system, but beyond that you will likely need to have the system serviced.

 

Pacifist's Kernel Extensions Report will display which extensions are those from Apple or from other sources.

 If the system does boot properly in Safe mode, and also is able to  load when using a secondary OS X installation or boot disc, then the  problem you are experiencing is likely with a third-party kernel  extension or other system add-on that you might have recently installed  or configured. One easy way to figure out if this is your issue is to  use the utility Pacifist. In its latest version it includes a kernel  extension scanning utility that will display information on the kernel  extensions installed on your system.

 Download Pacifist from the Charlessoft Web site,  and open it (you will have to wait 15 seconds for unregistered  versions), and then choose Display Kernel Extension Report from the  Pacifist menu. The program will scan through the Apple-related installer  receipts and match whether an installed kernel extension was included  with OS X or from a third party.

 Using this report, you can easily identify the non-Apple kernel  extensions and be able to remove them to see if the system will load  correctly with them uninstalled. Pacifist's report offers a Reveal in  Finder button that allows you to quickly locate the extension, or you  can try identifying the program associated with the extension and run an  uninstaller for it, if provided.

If you are unable to load the  system in Safe Mode but are able to boot to an external drive, then you  will have to take another route to address the issue.

 As a separate step in this instance, you can try identifying possible  causes to the problem by booting to Verbose mode, which will show which  drivers and system components are loading in the background, that is,  until the system loads or attempts to load the user interface. To use  Verbose mode, hold the Command-V keys together when the system reboots,  and instead of the grey Apple logo during boot you will see scrolling  text. Watch the text and if it pauses at a certain point right before  the system restarts, then try to write down any information about the  last few lines you saw before the error occurred, so you can convey them  to a technician or further investigate yourself to see if the problem  can be fixed. In some cases the output might reference a clear error  about a specific OS or hardware component, though in other cases the  output might be a bit more cryptic.

 

When you load in Verbose mode, the system will  display the text pertaining to each item loaded as the system starts up,  allowing you to potentially identify the step at which the system is  having trouble.

 If you are able to identify a specific kernel extension that is  causing the problem, then you can boot the Mac into Target Disk mode and  access its drive from another Mac via a Firewire connection, and then  browse the drive for the extension and remove it from the Extensions  folder.

 Unfortunately this type of approach does require you to have two  Macs, with both having FireWire connections, but is one way to remove a  faulty extension from the system so you can at least be able to boot it  again.

 If you cannot address the cause of the problem by uninstalling  applications or managing third-party kernel extensions and add-ons, then  a final approach would be to reinstall OS X over your current  installation, which can be done by simply booting to your OS X  installation volume (be it a Lion recovery partition or an installation  DVD). Follow the on-screen instructions to reinstall the OS. If you have  a Time Machine backup of your system, then you can optionally restore  to a backup made before the problem started happening (a preferred  route). If not, then a full OS reinstallation is the next best approach.  Doing this should keep all of your user data and installed applications  intact, and only replace the operating system files underneath  everything else on the drive.

Read more: How to address a constant reboot loop in OS X | MacFixIt - CNET Reviews