Mac OS: When and Why to Perform a Clean Installation

Clean installation refers to a method of preparing for and installing the Mac OS operating system such that the existing System Folder is disabled and a new one created. Links to additional information about performing a clean installation for different Macintosh computers and Mac OS operating system versions can be found at the end of this article.

What Is a Clean Installation?
A standard system software installation modifies and updates the existing System Folder. A clean installation disables the existing System Folder, leaving all files in place, and directs the Installer to create a new System Folder. The old System Folder will be renamed "Previous System Folder," and from there you can drag any fonts, preferences, or other necessary files to the new System Folder. You should drag as few items as possible, since some items in the previous System Folder may be the cause of your difficulties.

A clean system installation brings the system software back to the standard configuration. This is necessary when system software is unusable or has been modified, preventing a normal installation. It is also useful as a final step in troubleshooting.

When To Perform a Clean Installation
A clean installation may be appropriate in the following circumstances:

• when issues occur either during system startup or after startup is complete.
• when the Mac OS installer fails to complete.
• to fix a flashing question mark on startup.
• when the System Folder has become cluttered with unneeded or unfamiliar software.

In most of these instances, a clean installation should not be the first step in troubleshooting. The more premature a clean installation, the less productive it will be in identifying the source of the trouble.

Perform the following troubleshooting procedures to help you determine whether a clean installation is appropriate. Taking the time up front to locate the source of the issue will save you time in the long run.

Stops Responding During Startup

1. Hold down the Shift key while starting up the computer to disable extensions. If the computer fails to start up, there may be an issue with the core system software.

2. Start up from a known-good system disc such as the system software CD that came with the computer, a bootable floppy disk, or a bootable external hard drive. If starting up from another system disk succeeds, the system on the hard disk is unusable.

3. Perform a clean installation (links to instructions are at the end of this article).

Exception: If the computer starts up to a Type 41 error with extensions off, the issue is probably with the Finder alone. Instead of performing a clean installation, drag the Finder file from the System Folder to the Trash, then perform a regular installation over the existing System Folder.

Stops Responding While Using a Particular Program or During Program Installation

1. Start up from a known-good system disc such as the system software CD that came with the computer, a bootable floppy disk, or a bootable external hard drive. If certain extensions are needed to run the program you are testing, start up from a disk or CD that contains the necessary extensions.

2. Run the program to see if it crashes with known-good system software. If you cannot reproduce the troublesome behavior, your system software may be unusable.

3. Perform a clean installation.

Random Faults
There may be a fault with the core system software that affects system stability.

1. Start up from a known-good system disc such as the system software CD that came with the computer, a bootable floppy disk, or a bootable external hard drive. If certain extensions are needed to run the program you are testing, start up from a disk or CD that contains the necessary extensions.

2. Run programs one at a time to see if they stop responding with known-good system software. If you cannot reproduce the fault, your system software may be unusable.

3. Perform a clean installation.

Fault During Clean Installation
A Mac OS Installer fault may occur when a clean installation is needed.

1. Disconnect or turn off all peripherals except the keyboard, mouse, and monitor.

2. Perform a clean installation.

Flashing Question Mark on Startup

1. Start up from a known-good system disc such as the system software CD that came with the computer, a bootable floppy disk, or a bootable external hard drive.

2. Look in the System Folder for files named System and Finder. Some computers (for example, an iMac) also include a file named Mac OS ROM.

3. Use Find File to locate the files if they are not found in the System Folder.

4. Move the files back to the root level of the System Folder by dragging them onto the closed System Folder.

5. Restart the computer from the hard drive.

6. If the computer still fails to start up successfully, confirm that the System Folder is "blessed" by looking at the closed System Folder icon on the hard drive. If it does not have a small Happy Mac face or Macintosh computer icon superimposed upon it, open the System Folder, then close it and check again. If the System Folder icon still does not appear as described, use Find File to search for "System Folder". There should be only one System Folder on a Macintosh volume. If there is more than one on the same volume (not on different volumes), give the other System Folder(s) a different name, then restart.

7. If the computer still fails to restart successfully, one or more of the system files may be unusable.

8. Open the System Folder and double-click on the System file to open it. If it opens successfully, perform a clean installation; otherwise, drag the System file from the System Folder to the Trash, then perform a regular installation over the existing System Folder.

System Folder Cluttered
After many program installations and removals, the System Folder can become cluttered with extensions and other files that are of uncertain origin or that may no longer be needed. This creates wasted storage space and RAM. Clean installations performed for this reason are done for convenience, not necessity.


Related documents

58176: "Mac OS 8.5, Mac OS 9: Performing a Clean Installation"
26055: "Mac OS 8.0 & 8.1: Clean Installation Instructions"
22065: "Mac OS 8.0: Clean Installation Instruction for Performa Computers"
30034: "Mac OS 7.6: Clean Installation Instructions"
22048: "Performa w/Geoport: Restore Modem SW & Clean 7.6 Installation"
16095: "System 7.5: Clean Installation Procedure"