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When Safe Boot won't boot

Reader Simon Hansen finds his Mac slow to do his bidding. He writes:

My Mac has been running very slowly lately and I wanted to see how much faster it would be if I disabled its startup items. But when I try to start up in Safe Mode, it shows the progress bar, but that bar never fills up and my Mac doesn't boot. What's going on?

For the benefit of readers who are unfamiliar with Safe Mode (also known as Safe Boot mode), let's back up and explain how to access Same Mode and exactly what it is.

You enter Safe Mode by shutting down your Mac and then starting it up while holding down the Shift key as soon as your Mac's monitor lights up. You can let go of the Shift key when you see the Apple logo and a progress bar. According to Apple, this is what happens during Safe Mode (and I quote):

  • It forces a directory check of the startup volume. You may see a progress bar on the screen during this check, and the computer may take longer than usual to complete its startup.
  • It loads only required kernel extensions (some of the items in /System/Library/Extensions).
  • In Mac OS X v10.3.9 or earlier, Safe Mode runs only Apple-installed startup items (such items may be installed either in /Library/StartupItems or in /System/Library/StartupItems; these are different than user-selected account login items).
  • It disables all fonts other than those in /System/Library/Fonts (Mac OS X v10.4 or later).
  • It moves to the Trash all font caches normally stored in /Library/Caches/com.apple.ATS/(uid)/ , where (uid) is a user ID number such as 501 (Mac OS X v10.4 or later).
  • It disables all startup items and login items (Mac OS X v10.4 or later).
  • Mac OS X v10.5.6 or later: A Safe Boot deletes the dynamic loader shared cache at (/var/db/dyld/). A cache with issues may cause a blue screen on startup, particularly after a Software Update. Restarting normally recreates this cache.

Apple goes on to note that certain features won't work when your Mac is booted this way. Specifically, DVD Player, iMovie video capture, audio input and output devices, and modems won't work. If you're running Mac OS X 10.5 or later, hardware accelerated graphics are also kaput. And with OS X 10.6 or later, file sharing access is disabled so you won't be able to mount network attached storage (such as the drive in Apple's Time Capsule).

Now that we're all up to speed, why won't your Mac boot? I'm afraid my answer will be far shorter than the background information I've provided. I'll lay dollars-to-doughnuts that an external device (or many such devices) is causing the problem. I have multiple external hard drives jacked into my Mac Pro and I simply can't boot into Safe Mode--as with your Mac, mine stalls. However, once I shut down the Mac and unplug all those drives, my computer Safe Boots like there's no tomorrow. As you're booting into Safe Mode specifically to see how your Mac runs "naked," doing without those external drives for awhile won't hurt you.

And if you discover that your Mac is a speed-demon in Safe Mode but slows down again when you restart your Mac with no keys held down? Time to launch System preferences, choose Users & Groups, select your account, and take a close look at your login items. Do without those that you don't absolutely need and see how your Mac performs.

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