When a Mac first boots, it looks for a specific folder that contains the Mac OS System files; the folder name is stored in the boot blocks of the hard drive. If the computer looks in the designated folder and does not see the System and Finder files (or if no folder has been specified in the boot blocks), it will display a flashing "Question Mark Disk" -- this is more of a Mac OS 9 problem.

You can identify the System Folder in Mac OS X by looking at the root of the primary hard drive, and then looking for the folder with the blue X on it. The system folder contains all of the files that the computer needs to operate, and playing with the files inside can cripple your computer, but if you know what you are doing you can fix annoying problems that otherwise would be unsolvable.

OS X System Folder Components

Core Services

Core Services is the most critical folder in the /System/Library folder. Like the System folder itself, it has an X on it to denote its special status. It contains the BootX file, required for starting up from Mac OS X.

The folder also contains the Dock, Finder, Classic Startup, Help Viewer, and Software Update applications, as well as the login window process and the Menu Extras. There are also fake Mac OS 9 Finder and System files, created so that Carbon applications that expect to see these Mac OS 9-type files will "find" them. Yes, you will see the word Fake used in the Version description in the files' Show Info windows.

The SystemVersion.plist file determines what Mac OS X version is listed in the About This Mac window.

CFMSupport

CFMSupport contains software used for running Carbon applications. The CarbonLib file is in this folder, for example.

Extensions

This folder contains the kext (for kernel extension) files that load at startup, primarily acting as driver software for hardware peripherals. As their name implies, kext files are extensions of the basic kernel software that loads at startup. See the Extensions page for more information.

Fonts

This folder is one of several Fonts folders in Mac OS X. This one contains the fonts that are considered to be essential for Mac OS X.

Frameworks

Frameworks are an important component of Mac OS X, but you will have little reason to work with them directly in troubleshooting.

Briefly, frameworks are the Mac OS X equivalents of Mac OS 9's dynamic shared libraries, which means that they contain code that can be used by more than one application simultaneously. The basic idea is to eliminate the need to repeat code that will be used by multiple applications.

Frameworks have the structure of package files, although they appear to be ordinary folders and can be opened without the Show Package Contents contextual menu. A framework package can contain multiple versions of the shared software; applications that require the newer version can access it, and those that are incompatible with the newer version will be able to access the older version.

Frameworks can occur in other locations besides this folder. The ones in this folder are simply the ones that are most essential for the OS.

Preference Panes

This folder contains the panes that you access via the System Preferences application.

Printers

This folder contains files needed for printers to work with Mac OS X. Among other things, it contains the PPD and PDE files required for LaserWriter printers in Mac OS X.

QuickTime

This folder contains some QuickTime-related software, such as the QuickTime Updater application.

ScreenSavers

This folder contains the basic screen-saver options (Beach, Forest, and so on), which you accessed from the Screen Saver System Preferences window.

Services

This folder contains software needed for some of Mac OS X's Services feature, which allows you to access certain features of one application while you are in another application. You typically access this feature via the Services command in the menu that has the name of the active application (such as Finder, if Finder is the active application). If this feature is working, it allows you to open TextEdit with the selected text of your open application already pasted into an untitled TextEdit document, for example. This feature works only if the given applications support Services technology. Most do not, so most often, these options will be dimmed.

In the current context, Services also refers to some options that can be incorporated into any Cocoa application. As of Mac OS X 10.1, just two such Services are stored here: AppleSpell and Summary. AppleSpell, for example, allows a developer to include a spelling-checking feature in his application without having to write his own code.

Sounds

This folder contains the sound files (in AIFF format) that are listed in the Alerts tab of the Sound System Preferences window.

StartupItems

This important folder contains the various protocols that load at startup while you wait for the log-in window and Desktop to appear. These items include the Apache Web server, AppleShare, AppleTalk, Networking, and Network Time.

Blessing the System Folder (OS 9)

If your computer has "forgotten" which folder is the system folder, you may need to "Bless the System Folder".

Check to see if the folder is blessed:
Insert the Software Install CD into the drive (also known as Mac OS CD or System Software CD, but not the Restore CD), restart the computer, and hold the C key down as it boots up. The computer will then attempt to boot from the Software Install CD. Once the computer has successfully booted, open the boot drive and locate the System folder. If this folder does not resemble the "Happy Mac" icon the folder may need to be Blessed.

Re-Blessing the System Folder
These steps will help the computer recognize which System Folder to use:

1. Make sure your computer is booted from the Software Install CD.
2. Double-click on the hard drive.
3. Double-click on the System Folder
4. Double-click the System file. (It looks like a suitcase.) If you are unable to open the System file, you may need to reinstall the system software.
5. After the System file opens, close the System and System Folder windows and restart the computer.

If this doesn't work, try the following:

1. Boot up your computer using the Software Install CD.
2. Drag the System file out of the System Folder to another location on your hard disk (the Desktop is locked).
3. Close the System Folder.
4. Put the System file back into the System Folder and restart your computer.

If none of these steps work, you will need to reinstall you system software.