Unsupported System Software on the PowerBook 500 Series
Normally, the MacOS 8.6, 8.5, and 9.0 installers refuses to run on the PowerBook 500 series, even with a PPC upgrade card. Luckily, you can use a small gestalt trick to get the installer to run. Here, I'm only going to cover the PowerBook 500 series, since that's what I have.
Proceed at your own risk. Always back up anything important before trying this or any other system altering procedure.
Other Sites of Interest
Installing MacOS 8.5, 8.6 and 9.0
It isn't too difficult to get these unsupported versions running on a PowerBook 500 series. Before you start, you'll need the software listed below.
Save the software to your PowerBook's hard drive. Use of the NUpowr System Update is optional, but recommended.
Phase One: Run the Installer
The first hint you get that the installer doesn't like a PPC upgraded PowerBook 500 is clear: the installer refuses to start, and presents you with a nice error message referring you to read the documentation. To get arount this, download and install a copy of "Wish I were...". Set it to "PowerBook 5xx/PPC" and restart.
After restarting, run the installer or updater and go through the normal installation procedure. A full install can take some time, usually around 40 minutes. Make sure the PowerBook is plugged in, does not dim the display, sleep the hard drive, or go to sleep. If it tries, the installer might crash and you'll have to start all over with a clean install of a supported system.
Phase Two: The Reboot
Restart the machine when the installation is complete. Upon restarting, you will probably see the error message "This startup disk will not work on this Macintosh model. Use the latest Installer to update this disk for this model." Here's where we need to use the boot disk (or a boot CD, if you have an external CD drive handy) and ResEdit gestalt trick.
Boot your PowerBook from a boot disk and open ResEdit.
Phase Three: ResEdit the System File
Now that that you have access to the PowerBook's hard drive, use Res Edit to open the "System" file in your System Folder. Before chancing anything, read the steps below! Here's what you want to do:
1) Scroll down and find the resource named "gusd" and open it.
Now, reboot the PowerBook without the boot disk.
Phase Four: Success!
If all went well your PowerBook should be booting an unsupported system. After it finishes starting up, shut it down and reboot once more just to give it a clean cycle through a startup and shutdown.
Congratulations, you're now running new system software! If it didn't work you may want to try this procedure on a clean install. Other users who have tried this have reported success on a clean install when it didn't work the first time through.
Also, remember to disable any of the PowerBook's power saving features while installing, and use the power adapter, not battery power.
Updating to MacOS 9.0.4
The 9.0.4 updater won't work, and may crash. You will need to re-enable "Wish I were..." to get the updater to function properly. After you install the update, you'll note that your system still says it's still running 9.0.
The problem lies in the "PowerPC Enabler 9.0.4" system file not loading at startup time because it doesn't recognize your PowerPC upgraded PowerBook as a valid machine to run on. Unfortunately, the same trick to get the system to boot described above does not seem to work with the enabler file. The "System Resources" file was also not updated to 9.0.4.
Unfortunately, I have been unable to get 9.0.4 to work. It will install all of its related files, but still report itself as 9.0. It appears this is the end of the line for these old PowerBooks.
For other computers...
If you're not using a PowerBook 500 series, use this gestalt table instead of steps 3 and 4 above:
1) Look for a line that says "001F 001C"
Unlike the 8.5 hack to get it to run on unsupported machines, 8.6 does not appear to work on all of these same machines. See the "MacOS 8.6? On Unsupported 68K to PPC Upgrades" page for further information.