ARCHIVED: In Mac OS, how do I restart my computer with extensions off, and why would I want to?

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To restart a Mac OS computer with extensions off, hold down the Shift key during start-up. Keep holding it down until you see the Welcome to Macintosh dialog box with "Extensions Off" shown at the bottom of the box.

There are several reasons you may wish to start with extensions off:

If you are installing new software, some extensions may conflict with the software installer. This is especially true of virus detection extensions.
If you are having a problem with your computer, you can start with extensions off to see if it is an extension conflict.
If you want to open a file or program for which you do not have enough RAM, starting with extensions off may free up enough memory to open it.

Note: It is not possible to start Mac OS X with extensions off. However, you may run the Classic environment without extensions. To do this, follow these instructions:

From the Apple menu, select System Preferences....
In the System Preferences window, click Classic. If you can't see this icon, click Show All to reveal all your settings.
Click the Advanced tab.
In the "Startup Options" section, select Turn Off Extensions from the pop-up menu.
Click the Start Classic or Restart Classic button.