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From the book Reference 29.5

Reference 29.5
System Initialization Troubleshooting

Once you can identify the various stages—and know which processes and files are responsible for each, you are well on your way to diagnosing any startup issue. Case in point, the troubleshooting sections outlined here are organized by each stage of the system initialization process.

Also, throughout the remainder of this section, you will learn how each of the three primary OS X diagnostic startup modes—verbose mode, safe mode, and single-user mode—help resolve or identify issues. These three modes are initiated at the firmware stage but affect the remaining system initialization process at each stage. The ramifications of each diagnostic startup mode are covered with each stage throughout this section.

Troubleshooting the Firmware

Issues at the firmware stage are indicated by the inability of your Mac to reach the light gray screen with the dark gray Apple icon. The key to troubleshooting at this point is to determine whether this issue is related to the Mac computer’s hardware or system volume.

Serious Hardware Issues

If you don’t hear the startup chime or see the power-on light flash, the Mac hardware did not pass the POST. You may also hear a series of diagnostic tones or see a series of power-on flashes. If this is the case, your Mac has a fundamental hardware issue.

You can always check for simple things first. Is the Mac plugged into an electrical outlet? Are the keyboard and mouse working properly? Ultimately, a failure to pass the POST is usually indicative of a serious hardware issue. If this is the case, you’ll be best served by taking your Mac to an Apple Store or Apple Authorized Service Provider.

System Volume Issues

If your Mac passes the POST, but you are left with a flashing dark gray question mark folder icon, it means the firmware cannot locate a valid system volume or booter file. The Mac computer’s main processor and components are probably working correctly, and you may only have a software issue. Hold down the Option key during startup and use the Startup Manager to locate system volumes.

To troubleshoot system volume issues:

  • If the original system volume appears, select it to start up from it. If your Mac starts up from the system on the volume, open Startup Disk preferences to reset the volume as the startup disk. You can also attempt to define the startup disk when booted from another system volume like an external OS X Recovery disk, as covered in Lesson 4, “OS X Recovery.”
  • If the original system volume appears, but your Mac still cannot find a valid system or booter, you may need to reinstall OS X on that volume. As always, back up any important data from that volume before you make significant changes.
  • If your original system volume does not appear, the issue lies with that storage device. Start up from another system, like an external OS X Recovery disk, and use the storage troubleshooting techniques outlined in Lesson 13, “File System Troubleshooting.”

Troubleshooting the Booter

Issues at the booter stage are indicated by a flashing dark gray prohibitory icon—evidence of a failure to load the kernel.

To troubleshoot the booter:

  • If you’re starting up the Mac from a volume containing a system this Mac has never booted from, the prohibitory icon usually indicates that the version of OS X on the volume is not compatible with your Mac computer’s hardware. This is an extremely rare case that occurs mainly when a new Mac is restored using an older system image. The solution as this point is to reinstall OS X, using OS X Recovery, to a version of OS X that is known to work with this specific hardware.
  • Start up the Mac while holding down the Shift key to initiate a Safe Boot. The booter first attempts to verify and repair the startup volume, indicated by a dark gray progress bar across the bottom of the main display. If repairs are necessary, the Mac automatically restarts before continuing. If this happens, continue to hold down the Shift key. The booter verifies the startup volume again, and if the volume appears to be working properly, the booter attempts to load the kernel and essential KEXTs again. The booter uses the most judicial, and slowest, process to load these items and clears both the KEXT and font caches. If successful, the booter passes off the system to the kernel, which continues to safe-boot.
  • If the booter cannot find or load a valid kernel, you may need to reinstall OS X on that volume.

Troubleshooting the Kernel

Issues at the kernel stage are indicated by an inability to reach the login window or automatic login process, as evidence of a failure to load all KEXTs, the core UNIX system, and ultimately the system launchd process. If this is the case, your Mac is stuck at the light gray screen with the dark gray spinning gear icon. In these cases, the spinning gear progress indicator may stay visible indefinitely, again signaling a failure to complete the startup process.

To troubleshoot the kernel:

  • Start up the Mac while holding down the Shift key to initiate a Safe Boot. In addition to the Safe Boot procedures covered in the “Troubleshooting the Booter” section earlier, this forces the kernel to ignore all third-party KEXTs. If successful, the kernel starts the system launchd process, which continues to safe-boot. Completing the kernel startup stage via a Safe Boot indicates the issue may be a third-party KEXT, and you should start up in verbose mode to try to identify the problematic KEXT.
  • Start up the Mac while holding down Command-V to initiate verbose mode. The Mac shows you the startup process details as a continuous string of text. If the text stops, the startup process has probably also stopped, and you should examine the end of the text for troubleshooting clues. When you find a suspicious item, move it to a quarantine folder and then restart the Mac without Safe Boot, to see if the problem was resolved. This may be easier said than done, as accessing the Mac computer’s disk to locate and remove the item may not be possible if the Mac is crashing during startup. This is an example of where target disk mode really shines. As covered in Lesson 13, “File System Troubleshooting,” you can easily modify the contents of a problematic Mac system volume using target disk mode and a second Mac.
  • If the kernel cannot completely load while safe-booting or you are unable to locate and repair the problematic items, you may need to reinstall OS X on that volume.

Troubleshooting the System launchd

Issues at this stage are indicated by an inability to reach the login screen or log in a user (evidence of a failure by the system launchd process). If the system launchd process is not able to complete the system initialization, the loginwindow process does not start. Your Mac will be stuck with either a black screen or a white screen, depending on how far the system launchd got.

To troubleshoot system launchd issues:

  • Start up the Mac while holding down the Shift key to initiate a Safe Boot. In addition to the Safe Boot procedures covered earlier in the “Troubleshooting the Booter” and “Troubleshooting the Kernel” sections of this lesson, this forces the system launchd process to ignore all third-party fonts, launch daemons, and startup items. If successful, the system launchd process starts the loginwindow. At this point the Mac system has fully started up and is now running in safe mode. Completing the system initialization process via Safe Boot indicates the issue may be a third-party system initialization item, and you should start up in verbose mode to try to identify the problematic item.
  • Start up the Mac while holding down Command-V to initiate verbose mode. Again, if the text stops scrolling down the screen, examine the end of the text for troubleshooting clues; if you find a suspicious item, move it to another folder and then restart the Mac normally.
  • At this point you may be able to successfully Safe Boot into the Finder. If so, use the Finder interface to quarantine suspicious items.
  • While working in safe mode, you may also consider removing or renaming system cache and preference files, as they can be corrupted and cause startup issues. Begin by removing /Library/Caches because those files contain easily replaced information. As far as system preferences go, you can remove any setting stored in the /Library/Preferences or /Library/Preferences/SystemConfiguration folders you’re comfortable with having to reconfigure. A much safer solution is to simply rename individual system preference files in these folders. Once you have moved or replaced these items, restart the Mac, and the system automatically replaces these items with clean versions.
  • If Safe Boot continues to fail or you have located a suspicious system item you need to remove, start up the Mac while holding Command-S to initiate single-user mode. You’ll see a minimal command-line interface that allows you to move suspicious files to a quarantine folder. If you want to modify files and folders in single-user mode, you have to prepare the system volume. Start by entering /sbin/fsck -fy to verify and repair the startup volume. Repeat this command until you see a message stating that the disk appears to be OK. Only then should you enter /sbin/mount -uw / to mount the startup volume as a read-and-write file system. Once you have made your changes, you can exit single-user mode and continue to start up the system by entering the exit command, or you can shut down the Mac by entering the shutdown -h now command.
  • If the system initialization process cannot complete while safe-booting or you are unable to locate and repair the problematic items, you may need to reinstall OS X on that volume.
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