Mac Pro 2,1 which I hadn't updated since 10.9.1 because of the tedious nature of swapping the boot.efi. This approach was much easier after I was forced to take action when my system auto-updated (never a box I check...EVER!!!). I had recently grabbed an Aukey USB SSD drive-bay off Amazon for less than $10 for an external SSD drive for the girlfriend to quick add an SSD to her aging first-gen intel MBP. It worked well enough I ordered another for myself as it was much smoother than the Swiss Army Drive kit from Other World Computing [OWC] (though that gadget still holds a special place in my heart).
After the auto-update, I pulled the SSD drive which I've mounted into my tower via a kit from OWC (which if you aren't using an SSD in your 2,1 Mac Pro you have no idea how fast your tower can still run) and popped it into the Aukey tray. Through work I have a semi-current hardware MacBook Air, I booted to the SSD via System Preferences and rather tinker with the current 10.9.x version, I opted to jump from Mavericks to Yosemite.
I ran all the updates to get to 10.10.4. I then downloaded the appropriate web-build
https://forums.macrumors.com/thread...out-nvidia-pc-non-efi-graphics-cards.1440150/ for Nvidia. It wouldn't let me install initially, so I followed step #25 in the original post and made the appropriate changes for the web-driver that corresponded to 10.10.4. I restarted. Once up and running I set the Startup Disk to the MacBook Air's 10.8.5 system and restarted again.
At this point, I followed Mr. Zarniwhoop's Piker-Alpha instructions:
Another simplified installation approach is to use a second Yosemite-supported Mac and install Yosemite to the 2006/2007 Mac Pro's drive. This may be done either by attaching the 2006/2007 Mac Pro's drive as an external drive by placing the 2006/2007 Mac Pro in
target disk mode or
otherwise mounting the 2006/2007 Mac Pro's drive to a Yosemite-supported Mac. Then, after installation, copy Pike's EFI32 boot.efi to that drive's /usr/standalone/i386 and /System/Library/CoreServices/ directories overwriting the stock Apple EFI64 boot.efi and repair permissions. That drive should now be bootable on a 2006/2007 Mac Pro, although sometimes it still
may need to be blessed in the 2006/2007 Mac Pro.
All I needed to do was pop into terminal and make my everything visible via Terminal, I then dropped in the Piker-Alpha boot.efi and used Drive Utility to unmount the SSD from the MB Air, took it to my tower and plugged it in (via USB to confirm all was working as advertised), went to System Preferences and selected it as the System I wanted to boot and viola.