When a computer has Mac OS 8.6 installed, and a custom installation of a software component is performed from a Mac OS 8.5 software installer disc, the computer may not work properly or may not start up.

Your Mac OS 8.6 System Folder is no longer valid because your System file and other crucial items from Mac OS 8.5 and Mac OS 8.6 have become mismatched. To fix this, start up from another volume (such as your Software Installer disc) and run the Mac OS 8.6 Update again to re-synchronize the items.

The Mac OS 8.5 installer on the iMac and Power Macintosh G3 (Blue and White) system software discs opens an additional installer after the operating system is installed. This extra installer replaces the Mac OS ROM, System Resources and System file with versions intended for use with Mac OS 8.5. Installing any Mac OS 8.5 item from its installer still causes this extra installer to open. Unfortunately, this additional installer does not check to see if a newer item is installed and instead replaces the items that were updated in Mac OS 8.6. Although these three items are inadvertently replaced with older versions, the version of the Finder and other items installed with OS 8.6 are not replaced, so you end up with a non-functioning System Folder.

If you have a Mac OS 8.6 full install or update disc, you can start up from it and simply install the update to repair the System Folder. If you downloaded the update and it is still on your hard disk, you can start up from the Mac OS 8.5 disc and run the updater from the hard disk to repair the System Folder, too.