by (written by Jon, posted by Ed)

Travel is a rigorous thing on both people and computers. Over the last few weeks I have been made aware of, in a few PowerBooks, an issue where the units would kernel panic at boot. When these units were checked in for repair, it was noted in the symptoms that the machines had been traveling with their owners.

In most cases, we verified that the issue happened at boot, so we accepted the machines for full diagnostics. I was able to replicate one unit’s failure when booting from an external hard drive. In both cases, the owners felt that the computers had bad hard drives and were afraid that they would lose all their data.

So, following the simple steps of diagnosing, the next step is to Safe Boot the computers to see if that resolves the issue. Safe booting is a process on an OSX machine (10.2 or better) that disables kernel extensions (kexts) of third party applications and some native Apple kernel extensions. One disabled Apple extension is the Airport kernel extension.

In both cases, the machines would safe boot just fine. Knowing that the Airport kexts were disabled while safe booting, the next step was to entirely remove the Airport card. The machines would boot just fine with no Airport card installed. In further diagnostic steps, the Airport cards were inserted back into the PowerBooks and the units would boot normally. If issues are to arise after travel in a unit that you can remove the Airport card, removing it will often allow you to better diagnose the issue and repair the unit yourself.

Safe booting also forces a file system consistency check, the same thing as hitting “Repair Disk” in Disk Utility. Under 10.4, safe booting also disables nonessential fonts and all login/startup items. Because all these things are disabled, one cannot enjoy some functionality of the machine: applications that will not work while safe booted include DVD Player, iMovie (video capture), Airport, various input/output devices, and modems.

To safe boot your computer, simply hold down the shift key immediately after the startup chime. It may take a while for the computer to say Welcome to Macintosh, but that just means the safe boot process is under way.

There are plenty of other key combinations that modify startup behavior, and each has its own diagnostic merit:

  • Holding down X on a PowerPC-based machine with OS 9 installed will force the machine to boot from the OS X volume.
  • Holding down apple-option-shift-delete will force the computer to ignore the internal hard drive, and look for other startup devices like the install DVD that came with the computer or a bootable FireWire external drive.
  • Holding down V will force the computer to boot in verbose mode. Instead of the friendly Welcome to Macintosh screen, you’ll see a black background with white text. This is nice when you have a machine that won’t boot completely but is hanging at the same point every time.
  • Holding down apple-S will put the machine into single-user mode, a Graphical User Interface (GUI)-free mode that allows access to the UNIX underpinnings of OS X. You can move or delete files here, run fsck (file system consistency checks—also done on safe booting), and lots more. Be careful, though, it’s very easy to make things worse here, so stay away unless you’re comfortable. If the computer hangs up at some point in verbose booting, you can often move or delete the culprit in single user mode.

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