As part of the troubleshooting process, it often makes sense to attempt booting a machine in Safe Mode by holding down the shift key immediately after pressing the power button. By holding this down until the gray apple with spinning gear appears, many computers that wouldn’t boot will magically start right up.

Many things happen when you start up in safe mode. Firstly, the startup disk undergoes a directory check and any problems found that can be repaired are repaired. This step alone solves many startup issues. In safe mode, only the kernel extensions absolutely essential to basic operations are loaded at startup. These extensions can be found in /System/Library/Extensions. Under Tiger, all fonts other than those in /System/Library/Fonts are disabled, all font caches are moved to the trash, and all startup and login items are disregarded.

A customer came in recently with a 17” PowerBook G4, the very last 17” model with the high resolution screen. She bought it used for basic e-mail, web browsing, and social networking uses because she was drawn to the huge screen and portability, but mere months after the Craigslist transaction the computer showed intermittent failure to start all the way up.

The first thing we did after it was checked in was try to boot it in safe mode, and lo and behold the machine started right up. Next step was to try starting it off of a known good external drive with a known good installation of OS X 10.4 Tiger. It failed to boot from this FireWire drive, suggesting a hardware problem.

Apple makes available some diagnostic software to its service providers. While it looks impressive and apparently runs tests on all hardware components of all Apple machines, the unfortunate reality is that the software is generally useless: it fails to detect even the most basic memory failures or graphic processor wonkiness in most cases. However, in the extremely rare case that it does find a failure, it’s a definitive diagnosis to the part in question.

In this very rare case, the diagnostic software threw a cryptic failure code for the graphics architecture on the logic board. When the drivers for this card load during normal startup, the bad component causes the whole machine to freeze. Safe boot prevents the card from seizing the computer. The failure to boot from an external known-good startup disk (like the restore media that came wtih your computer) confirms that this is a hardware problem.

The cost for replacing the logic board in this machine is about four hundred dollars, and is done on a mail-in basis with Apple (we can do it in-house, but Apple’s flat-fee repair program is more economical by about $800!), and the customer didn’t want to spend that money.

The trouble with Safe Mode is that it disables pretty much everything, including AirPort wireless networking. On a laptop, that’s a real drag. It’s been many years since I’ve been to a cafe with ethernet cords strewn about the floor, and being tethered is just no fun. Under Tiger (sorry, Leopard users!), simply boot up in Safe Mode, launch Terminal, and issue these commands:

sudo kextload /system/library/extensions/appleairport.kext
sudo kextload /system/library/extensions/appleairport2.kext

Enter your password when prompted, then fire up the Network Preference Pane from System Preferences. Choose to show AirPort in the Menu Bar, turn it on, select a network, and go about your business.


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