I basically followed this:
http://donaldbjames.com/yosemiteonmacpro.html
Except that I used an 8GB usb thumbdrive to hold the Yosemite installer + PikeYosFix.pkg.
After the second reboot, the startup screen is black instead of gray behind the silver Apple logo + progress bar — that tells you that it worked.
When finished, the computer reported OS X version 10.10. I ran App Store updates afterward, and it saw some printing stuff, the Yosemite Recovery update, and the 10.10.5 Combined Update. It seemed to hang for EVER on the Recovery one. I finally ended up running Software Update from the cli:
sudo softwareupdate -i -a
And left it overnight. In the morning, it said “Done” but it appeared to have ONLY done the Recovery update. All the other updates were still in the list when I ran another check, and it seemed to complete them, but after reboot it was still on 10.10 and still wanted all those updates again. Weird!
So I downloaded the 10.10.5 combo updater from Apple separately, ran it, it worked. Then I did the other updates (printer drivers, Safari, iTunes, etc.) the normal way via the App Store, and everything seems fine now. (shrug)
Thanks for the clear step-by-step instructions, Don!
—MB September 2016