To start up into Safe Mode (to Safe Boot), do this:
# Be sure the computer is shut down.
# Press the power button.
# Immediately after you hear the startup tone, hold the Shift key. The Shift key should be held as soon as possible after the startup tone but not before.
# Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).
* During startup in Mac OS X v10.4 or later, you will see "Safe Boot" on the login window, which appears even if you normally log in automatically.
* During startup in Mac OS X v10.2 through v10.3.9, you will see "Safe Boot" on the Mac OS X startup screen.
To leave Safe Mode in any version of Mac OS X, restart the computer normally, without holding any keys during startup.
[http://support.apple.com/kb/TS1411#symp1|Info sourced from Apple support forum]
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You can also be more proactive:
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# Try your install CD that came with your Mac, reboot, hold down the letter C after you hear the chime, and when it boots to the installer, pick a language, and then run Disk Utility by finding it in the top menu.
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# Instead of holding down C at startup, hold down the letter D and make sure all your hardware's okay.
Can you start it in Safe Mode?
To start up into Safe Mode (to Safe Boot), do this:
# Be sure the computer is shut down.
# Press the power button.
# Immediately after you hear the startup tone, hold the Shift key. The Shift key should be held as soon as possible after the startup tone but not before.
# Release the Shift key when you see the gray Apple icon and the progress indicator (looks like a spinning gear).
* During startup in Mac OS X v10.4 or later, you will see "Safe Boot" on the login window, which appears even if you normally log in automatically.
* During startup in Mac OS X v10.2 through v10.3.9, you will see "Safe Boot" on the Mac OS X startup screen.
To leave Safe Mode in any version of Mac OS X, restart the computer normally, without holding any keys during startup.
[http://support.apple.com/kb/TS1411#symp1|Info sourced from Apple support forum]