PASSED-Ran the following command from the Terminal to get the board id and saved the it on the iMac:
ioreg -lp IOService | grep board-id
PASSED-Connected mac3,1, via firewire, to my 2010 iMac which is Sierra supported.
PASSED-Downloaded Sierra on the iMac
PASSED-Booted mac3,1 in target disk mode; both drives showed up on the iMac.
PASSED-Launched the Sierra installer.
PASSED-Selected the disk "extra" as the target for the installation.
NOT SURE-The installer started and after copying some files the iMac restarted and went directly into the installer. The installation continued without error until it finished at which time the iMac restarted again.
Things started as described, however, I stepped away, and I'm not exactly sure what I returned to find.
NOT SURE-The restart caused the iMac to run Sierra (using the "extra" drive from mac3,1) and was waiting for me to enter a password to continue. At this point I shutdown both computers. I restarted the iMac and held down the Option key which allowed me to select the iMac's normal start up disk. Once the iMac restarted I once again restarted mac3,1 in target disk mode.
My Mac Pro actually shut down (at some point), not restarting as described above. And, my MacBook Pro3,1 remained in Target Disk Mode. When I started the Pro, I didn't seem to have to change the Startup Disk.
FAILED (miserably)-Once the drives were again visible on the iMac I used the Terminal to edit the PlatformSupport.plist file. The command I used was: sudo view /Volumes/extra/System/Library/CoreServices/PlatformSupport.plist
The view command is a variant of Vi so you'll need to know Vi to edit the file. I added a line for the board id for the value we got from the command we ran earlier and a line for the model properties using the value of MacPro3,1.
Of course the command:
sudo rm /Volumes/[Main Drive Name]/System/Library/CoreServices/PlatformSupport.plist
can work just as well.
I could not, for the life of me, edit the .plist file, which appeared as Locked in TextEdit. And I "googled" and tried everything I could possible find. This command thing is not really my forte, and so, even though I understand the adding a line concept, it didn't happen, as much as I would have liked things to work.
Any words of wisdom?
Could it be that my MacBookPro3,1 doesn't have the necessary processor capabilities to handle the upgrade to Sierra? Someone mentioned the following, which nobody else seemed to touch upon: "The MacBookPro 3,1 isn’t supported by this patch, due to the CPU not supporting SSE4.1." Unfortunately, I'm not really sure what it means.
So close, yet so far.
NEVER GOT HERE-After editing and saving the PlatformSupport.plist file I shutdown mac3,1. I then restarted mac3,1 while holding the Option key which allowed me to select the drive "extra" to startup from. The computer started and asked me for my password after which the setup program started and eventually I was in macOS Sierra.
So that's it. This is the same process I used for my 2008 Xserves for OS X 10.9 and 10.10 and should work with some of the other computers that are now no longer supported. Enjoy[/QUOTE]