Hello all -
I feel Jarvil's pain. I have been up literally ALL NIGHT trying to do something that ought to take no more than 20 minutes - just installing Windows 8.1 through bootcamp on my new Mac Cylinder. No matter what, I got the dreaded 'Windows Can't Modify the Boot Record' yadayadayada - and I have tried every solution on this board (and I think every other discussion board here at Apple).
Well, guess what? It turns out that there's an easy solution to this "Windows Can't Modify the Boot Record" problem, but it's not listed here. Shockingly, it's from Microsoft. The steps you need to follow can be found at this part of the Windows Knowledge base, but here are the simplest instructions I can come up with for this ridiculously complicated workaround (thanks, Apple.) This is the *only* solution that worked for me, but it did work.
Please Note: The instructions below presume that you have an actual bootable installation DVD of Windows 8.1 and a valid product key; that you've made an EFI USB stick with a Windows ISO and appropriate bootcamp files, as instructed, have partitioned your drive using Bootcamp, and have had a failed installation, with Widows returning the error that the Master Boot Record couldn't be modified and that installation cannot continue (or something to that effect).
Once this happens:
1) Click "OK" when in the error message where windows tells you that it cannot continue. This will effectively shut down your Mac.
2) BEFORE you restart your machine:
a) Make sure you have a DVD of the Windows 8.1 installation disc (iso will not work; you need the actual DVD)
b) Make sure you have a DVD reader of some kind attached to any USB port, with the Windows installation disc in the DVD drive.
c) Remove the EFI USB stick you just used that resulted in the failed installation.
3) Turn on the computer, hold down 'option' and select the 'EFI' image of the DVD that will appear to the far right of your screen. You'll get a black screen with the 'Press any key to boot from CD / DVD message.' Press any key and windows installation will begin again.
4) Follow the prompts until you get to the screen that says 'Install Now.'
5) Do not click 'install now.' Instead, on the 'Install Now' screen, look to the left corner to find the option 'Repair Your Computer.' Click that.
6) Select 'Troubleshoot' from the "Choose an Option" screen
7) Click 'Advanced Options' from the 'Troubleshoot' Screen
8) Click on Command Prompt
9) Execute the following commands, one at a time, making sure that each command returns 'Completed Successfully' (and they all will):
Bootrec /fixmbr
Bootrec /fixboot
Bootrec /scanos
Bootrec /rebuildbcd
10) Once you've executed these commands successfully, 'back out' of the Windows 'Repair My Computer' menu until you see the option 'Quit and return to Windows 8.1' Click that. Installation will resume immediately, and - shockingly - you will see Windows installation move to the next phase of installation, 'looking for devices.'
From that point, installation will work just as you'd expect it to. There's just one thing. As others have pointed out, each time Windows asks to restart, you need to be standing by to hold down the 'Option' button. At this point, until Windows installation is completed, be sure you select the Windows drive (and not the EFI installation disc) every time Windows requires restart.
Fifteen minutes later, you'll have Windows 8.1 up and running. Install the latest bootcamp drivers from the USB stick you created earlier, restart one last time, and finally, you're all set.
I really hope this saves someone the (nearly) fourteen hours it took me to complete this installation properly. Let me know if it helps.
Cheers, Keith