Just thought I would add my 2 cents for other people searching for solutions:
I have a MacPro (new, cylinder) with an external thunderbolt enclosure with 4 drives inside (an NTFS-formatted SSD with Windows, an NTFS hard drive for Windows files, and 2 hard drives formatted for OS-X). The internal MacPro drive SSD contained OS-X.
In the end, I finally got things installed after getting the same error message by:
1. Disabling BitLocker on the Windows boot-drive SSD (not sure if this was necessary). I left it enabled on my other non-boot windows drive.
2. Changing the boot priority to start with the Windows boot-drive (I couldn't get the drive to show up in "Startup Disk" until I disabled BitLocker, but this may have been unnecessary)
I did the above first, hoping that assigning the Windows boot drive as first priority would fix things - it didn't. So I then removed all drives with EFI-partitions other than my Windows boot drive. This meant.
3. Open the enclosure and removed the 2 OS-X formatted drives (they all get EFI partitions, apparently). I kept the 2 windows drives installed.
4. Open up the MacPro and removed the internal SSD.
After this... Finally everything worked, like magic - except that my keyboard wasn't working properly (function keys like volume control weren't working). So I...
5. Re-installed bootcamp 6 drivers (a repair install option will be available).
Finally, to get everything back the way it was:
6. Reinstalled all drives and re-actived BitLocker on the windows SSD.
Windows 10 is nice, I think a worthwhile upgrade, but there has to be a way to make installation smoother...
Cheers!
FYI UPDATE:
I recently encountered the same error message installing Windows 10 on a 2012 Mac Mini Server:
0x80073892-0x20009
The installation failed in the SAFE_OS phase
with an error during PREPARE_ROLLBACK operation.
I did some new research and discovered many users were having this problem, usually on PC desktop machines. There were many convoluted suggestions, even from Microsoft Tech Support, but seldom did they seem to fix the problem. Then I ran across one post that fixed it. For users with 2 drives (SATA) in their system, disconnected the one not being installed to. I did this, and the Windows 10 upgrade completed flawlessly.
On this system I had replaced one of the hard disks with a 512GB SSD partitioned in half for Windows, and the other half as part of a DIY Fusion OS X drive with the remaining hard disk (the one I had to disconnect temporarily). On another similarly configured but NON-Fusion Mac Mini, I initially encountered the same error message, but I decided to "clean install" the upgrade without saving any data or programs and that seemed to work. Apparently the "PREPARE_ROLLBACK" wasn't an issue on a clean install.
Not a very robust installer program released with this update by Microsoft!