Extensions Manager: Extensions Off Compared To Shift Down When You Start Up
Holding the Shift key during startup instructs Mac OS to not load any item that is in the Extension folder, Control Panels folder, or the Startup Items folder. Holding the Shift key during startup does not cause any item to move. The Extensions Manager control panel, however, does physically move items around, but only listed items. When you use the "All Off" set, the Extensions Manager control panel moves all listed items to their respective "Disabled" folder, such as Extensions (Disabled). Items that are not listed in the Extensions Manager control panel are not physically moved, and therefore still load during start up.
Some control panels, including the Startup Disk control panel and the Extensions Manager control panel itself, can be opened after starting up with the "Shift down" method.
The Extensions Manager control panel can also be accessed before most extensions load by holding the Space Bar during start up.
For example, the Extensions Manager switches off Chooser extensions, but holding the Shift key down during restart does not. The Extensions Manager does not display or switch off all extensions, whereas the Shift down restart does disable all extensions.
When you are trying to isolate an extension conflict and are not sure if it is an Apple System extension or a third party extension, restart while holding down the Shift key.
Few third party software items use extension types that the Extensions Manager does not recognize and turn off in the All Off set. If you cannot start up with extension disabled by holding the Shift key, start up from a Mac OS CD or floppy disk. It may be necessary to reinstall your system software, or perform a clean installation of it. If you cannot start up from a Mac OS CD (or floppy disk), there may be an issue with a hardware device attached to your computer, or with an internal hardware upgrade.