I've tried to re-install Windows 7 using Bootcamp Assistent, since Windows 10 gave me a lot of headache.

Ran in to the error "No bootable device detected...", so I used following steps:

  1. Make EFI bootable USB installer

    • Download Win10 install ISO
    • Boot into Windows DVD setup to access a Windows command prompt
    • open command prompt at install screen by pressing shift+F10
    • load diskpart and make clean install disk by typing:

      diskpart
      list disk
      select disk n #n=USB stick
      clean
      convert gpt
      create partition primary
      format fs=fat32 label=Winstall quick
      exit
      exit
      
    • mount downloaded install ISO

    • drag everything from the mounted ISO onto the USB stick
    • delete bootmgr and bootmgr.efi from the stick to avoid BIOS/CMI boot and force EFI boot
  2. Flash MacPro4,1 with MacPro5,1 firmware

    Bootrom MP41.0081.B07 (EFI 1.4) to Bootrom MP51.007F.B03 (EFI 1.5) (If your machine shows Bootrom version MP41.0081.B08 you could first try to continue without upgrading. I don’t know which Apple EFI version is contained in there. Downgrading should be possible as well, definitely possible to version MP41.0081.B07. Please research if you don’t want to lose the downgrade possibility to MP41.0081.B08) The upgraded EFI finally enabled me to complete the boot process and you will have all other advantages of the 2010 and later models (hex-core CPU, faster RAM, Audio over DP) This upgrade is reversible. (If using El Capitan you need to disable System Integrity Protection. Otherwise the flash will not perform. If patch is running successfully you can see a big grey loader bar and the apple logo. Optical drives will open.)

    • Download MacPro5,1 firmware here -Mount .dmg (to prevent error 5570)
    • Download Mac Pro Firmware Utility here
    • Run Utility and follow instructions
    • check upgraded bootrom version and identifier in System Information
  3. Setup Windows with EFI Boot
    • remove ALL physical drives except the to be Windows drive
    • plug in Winstall USB Stick
    • Boot Mac holding option/alt key
    • Choose EFI Boot
    • Enter setup and select GPT target disk
    • Install
  4. Install Bootcamp Drivers (optional)
    • Boot into Windows
    • Download Bootcamp Support Software
    • Run Setup

The problem is, however, that my machine gets stuck in a bootloop during Windows 7 install... After initial installation, it tries to reboot but gets stuck at grey screen and keeps rebooting. When upon rebooting while holding option/alt key, the Windows drive does not appear, only the 'Winstall' USB stick.

Is this a Windows 7 issue and should I try Windows 8/8.1?

I plan to use drives as followed:

  • 640 GB HDD (old OS X install)
  • 500 GB HDD (Windows7)
  • 240 GB pcie SSD (Main OS X)
  • You have expended a great deal of effort to fail at trying to accomplish a foolish task. Install Windows 7 using the BIOS boot method. What do you gain by using an EFI boot method? – David Anderson Aug 30 '16 at 15:06
  • Because, when I install win7 using BIOS method (i.e. via Bootcamp Assistent), it won't boot when I have my pcie SSD installed... Which is very odd, because I had Windows 10 (upgraded from Win7) installed and it booted fine with all drives present. I assumed above method was suitable for Win7, but I'll give installing Win8.1 a try tonight. – Mr. Vermeer Aug 30 '16 at 20:03
  • Fine, then install Windows 7 on the 500 GB HDD and BIOS boot Windows 7 from whichever drive is disk0. – David Anderson Aug 30 '16 at 20:58
  • When I first installed 64 bit Windows 10 on my 2007 iMac, it was an upgrade from Windows 8.1. This did not work very well, so I did a clean install of Windows 10. This worked much better. Also, there is a new Windows 10 just released this month. Have have yet to try this new version on a Mac. Runs well on my HP computer in EFI mode. The 2007 iMac uses a BIOS boot of Windows. – David Anderson Aug 30 '16 at 22:14
  • As for your questions. Yes, running Windows 7 in EFI mode on any Mac is a huge problem. Apple has stated this can not be done. The posts, I have read where anyone actually succeeded, list problems getting the Boot Camp Support Software to work properly. I would not recommend Windows 8. This version was a complete disaster. Windows 8.1 is better, but I still consider this be a train wreck. Basically, Microsoft tried to merge its other technologies into Windows to quickly and released both 8 and 8.1 before either version was ready. – David Anderson Aug 30 '16 at 22:23

"Windows 7 x64 (64-bit) will NOT boot natively using UEFI on a Macintosh!"

See source with technical details here: https://darobins.wordpress.com/2009/10/04/native-uefi-windows-7-boot-on-mac-mini/

PS It would have been nice to not only copy and paste my installation guide in your question. That cost me some time to put online. A link or reference as source would have been nice. From the title of my post it could have come to your mind as well, that a WIN8 or WIN10 installation had been the goal, not WIN7...EFI Boot Windows 8.1 or 10 Install on MacPro4,1 / Mac Pro (Early 2009)

Well, the answer seems to be installing Windows 8.1 instead of Windows 7. Windows 7 doesn't run properly in EFI boot mode.

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