Extensions let you to customize your Macintosh by extending or enhancing your system software. The term "extensions" refers to both system extension files (e.g., Apple CD-ROM or File Sharing Extension) and control panel files (e.g., Macintosh Easy Open or Monitors & Sounds). Most extensions are installed by software. Sometimes these extensions can conflict with each other by using the same system resource or by providing contradictory instruction to your computer. Your system can also not be compatible with certain extensions.
Resolving an Extension Conflict
Mac OS X
Restart your computer and when you see the "Welcome to Macintosh" screen, press and hold down the space bar. Continue holding down the space bar until the Extensions Manager appears. Unselect all of the extensions, and then select only the extensions that are absolutely necessary for your system to start up; most programs will need the Apple CD-ROM and the Quicktime extensions.
Click and hold down the SETS button at the top of the box, then click on SAVE SET. Give the set a name like "Troubleshooting Config" so you can use it again later.
When done, click on the box in the top corner; your Mac will start up normally. Check and make sure your system starts up correctly and then redo the previous steps, only this time adding more extensions to the system; make sure to save the settings configuration so that you can easily keep adding to it. When your system problem reoccurs, you have found the extension that is causing the problem. Contact the manufacturer to obtain the newest version if they have one available.
Mac OS 9
Restart the computer and immediately press and hold down the Shift key on the keyboard as it boots up. If it boots successfully with extensions off, this indicates an extension conflict. Most likely, moving the last extension installed into the "Disabled Extensions" folder will solve your problems. If this does not solve your problem you will need to isolate the extension that is causing the problem.
An extensions manager (e.g., Apple Extensions Manager) enables you to selectively disable extensions. Keep in mind that any extension you use to disable other extensions may itself be contributing to the problem.
Selectively re-enable your extensions to isolate the one(s) causing the problem. You can re-enable them by using an extension manager or by manually moving them back into System Folder
Re-enable one or a few extensions, try to re-create the problem, then repeat this process. For example, you can begin by re-enabling those extensions included with your system software and then re-enabling your favorite extensions.
Keep track of which extensions you just enabled. If the problem recurs after you re-enable an extension, leave that extension enabled, and disable all other extensions so your system and application are running with only that extension enabled. If the problem recurs after you restart, you've found the problem extension. If the problem doesn't recur, you'll know it is caused by a combination of extensions, and not that single extension. You can troubleshoot further by re-enabling other extensions to determine which combination is the cause.