Mac Geekery

Answer

Startup troubles generally boil down to three things: Startup Items, non-standard launchd items, and Kernel Extensions. Not so incidentally, that’s exactly what Safe Mode disables.

Startup Items

Startup Items can be found in /Library/StartupItems and in /System/Library/StartupItems. Either can be moved (perhaps requiring sudo) to something like StartupItems.orig and a reboot will tell you if anything therein was causing the trouble. I’ve never known the System tree Startup Items to cause any trouble, but you never know. No reasonable application should be installing or even modifying anything therein, leaving us to worry mostly with the Library tree.

Launchd

If you’ve crafted any launchd items, they can mangle up the boot process sometimes, too, and Safe Mode disables them. To conclusively rule out launchd-related fun, remove items in LaunchAgents and LaunchDaemon folders you find in ~/Library and /Library.

Kernel Extensions

Safe Boot disables all third party drivers and a handful of Apple-supplied drivers. Extensions, both Apple and otherwise, are stashed in /System/Library/Extensions. You can guess what most do based on their name. smbfs.kext provides SMB filesystem access, NVDAResman.kext provides functionality to NVIDIA graphics cards, and so on.

If you’ve recently (or ever) installed a device driver for some hardware you’ve added to your machine, or tried to install third party drivers for standard hardware, or what-have-you, try hunting down its kext file and remove it. After any change, touch /System/Library/Extensions to trigger the OS to rebuild the kextcache.

$ sudo touch /System/Library/Extensions

In theory, changing the contents does this automagically. Somehow, removing things triggers this less reliably than adding them. Best to be safe and do it manually. Especially since we’re in territory where we really, really, really want to be sure that things are being updated appropriately.¹

Fun With Hardware

If you’ve just done an erase and install, and you’re still having the problem, playing with Startup Items and Kernel Extensions won’t really get you far. You’re more likely looking at a hardware problem.

Start by removing all PCI cards and other non-essential hardware like Airport cards, bluetooth adapters, and modems, as well as any extra RAM, and see if the machine boots. If it does, congratulations! You’ve just isolated the problem to one of the removed devices. Next, start adding them back in one at a time until the problem comes back. Whatever you add that triggers the boot problem is the culprit. Given your symptoms, I’d bet even money or better that it’s an Airport card.

If you remove all this and it still doesn’t boot, you get to dig deeper. Try removing the hard drive and boot to a clean OS on a FireWire² drive. Try removing the graphics card, after an appropriate amount of time, try to SSH in and see if it finished booting. I’ve had to get a machine down to a power supply and logic board – and nothing else – before. Yes, it’s tedious. Yes, you should appreciate your local Mac Geniuses more.

¹ I’d go as far as actually deleting the Extensions.kextcache file when I had reason to think touching wasn’t working, either.

² You can use a CD here, but only as negative proof. The CD’s boot OS lacks a lot of drivers that the full OS includes. If it fails here, things are busted; if it works, you can’t really be sure.