This guide is how we got our MacPro 1,1 running the retail / golden master of Mountain Lion OSX 10.8.
You may have a different type of unsupported Mac, and our method may work for you. But again, this is how we got Mountain Lion working on a Mac Pro, which is the only unsupported Mac we tried this installation method with.
Mountain Lion capable Mac (Macbook Pro 8,2 in our case)
Firewire 800 cable (400 should be fine too)
Our trusty MacPro 1,1. (We updated from 10.6 Snow Leopard.)
*** please note the upgraded graphics card. The Mac Pro will not boot with a 512MB stock card, like Nvidia 7300GT, or ATI X1900XT. We upgraded to ATI Radeon 5770.
What we did was use a capable Mac to install onto the MacPro in target disk mode. There are several other installation methods out there, but they involve working with more files to build your own installer, or using a PC boot-loader. We are lazy and chose Firewire instead.
Connect the Firewire cable from the capable Mac to the Mac Pro.
Power on the Mac Pro, and press the “T” key to boot into Target Disk Mode.
Your Mac Pro monitor should have the Firewire logo floating around on the screen, and the Mac Pro’s HD should be mounted as external disk on your capable Mac.
Go into System Preferences, and select your Mac Pro (firewire’d) drive as the Startup Volume, and choose to Restart.
After restarting, double click the “Install OS X Mountain Lion.app”, and install the new OS onto the target volume. This process works just like any other Mac OS installation, and the computer will restart once complete.
After the re-boot you should be looking at the new Mountain Lion OS. But don’t get too excited yet, cause there is still more work to do.
Finish the Install:
This is the more advanced part of the installation. Go into System Preferences and select the capable Mac’s system (not the newly installed Mountain Lion partition) as startup disk, and then restart. At this point we want to be able to modify files on the Mac Pro without them running.
This should boot you back into your non-Mountain Lion partition.
Now power off the computer one more time.
Turn it back on while holding the Option key, and you should be able to choose a startup volume. Select the Mountain Lion partition.
IT WILL TAKE A WHILE TO BOOT. You should see the apple logo on the screen for several minutes with the spinning wheel. Just let it go, and you should be looking at Mountain Lion after a bit. Be patient.
iCloud may not function right away, so if you would like that OS feature, here’s how to get it working. We learned of this fix for iCloud from HackerWayne’s post at Mac Rumors.
Using a capable Mac with Lion or Mountain Lion, create a new user account with full admin privileges, and iCloud enabled.
Use Migration Assistant to transfer the account over to your Mac Pro.
Once prompted of what you want to transfer, just select this new account. You do not need Applications, computers setting, or anything else. Just the user account.
After the transfer is complete, you should be able to use iCloud on the Mac Pro. You can remove the “dummy” account you just created from both computers.