Boot Windows 7/8 from GPT on BIOS system : No hybrid MBRs or DUET!

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  1. Posts : 72
    Windows 7 Home Premium x64
       #1

    Boot Windows 7/8 from GPT on BIOS system : No hybrid MBRs or DUET!


    Hi guys,

    Till now, Windows 7/8 x64 could boot from a GPT disk only on a UEFI system. Methods to enable this are hybrid MBRs and DUET. Both of which are hacks.

    We now have a simple way to boot windows (BOTH 32bit AND 64 bit) from a GPT disk on a BIOS system WITHOUT hybrid MBRs or DUET, using just a small MBR disk to hold bootmgr, can be HDD or USB stick. It can even be a floppy :) It will just hold the boot files.

    GPT is preferable for several reasons, independent of UEFI :

    1. No 2TiB limit on Hard disk size
    2. No distinction between primary and logical partitions, and the associated space allocation decisions.
    3. Partition table is redundantly stored, checked using CRC
    4. No conflict of partition types
    5. No need to use the logical partitions which are implemented in an EXTREMELY error-prone manner.

    The principle here is simple: Have a ready Windows installation on a partition on a GPT disk. Then add the boot files and boot record to an MBR disk, and add a bootloader entry to the BCD pointing to the Windows installation (more precisely, winload.exe)

    First install windows on a GPT disk. This can be done in several ways :


    1. Install in BIOS mode on MBR and then convert using gptgen
    2. Install in UEFI mode using DUET. (ONLY for 64-bit!)
    3. Apply the windows install image (install.wim) using ImageX (Windows 7) or DISM (Windows 8+)on the GPT disk (best)

    Next insert the Windows DVD, and that small disk, whatever it may be. Open the command prompt.

    Use diskpart to verify for yourself which drives are data drives, and which is your windows boot drive. For safety, I recommend removing the drive letters of all other drives except the windows boot drive, which I'll refer to as C:, using the command :
    Code:
    remove
    Type the following commands in diskpart. I'll refer to the other disk as Disk 1. It should be self-explanatory

    Code:
    select disk 1
    clean
    create partition primary [size=##]
    format
    select partition k
    active
    list volume
    select volume n
    assign
    list volume
    exit
    Note down the letter assigned newly in the last list volume. Assume it is D:

    Install boot files to the drive
    Code:
    bcdboot C:\Windows /l en-us /s D:
    Install the bootsector
    Code:
    bootsect /nt60 D: /mbr /force
    Edit the BCD for a small tweak (this may not be necessary : experts advice looked forward to)
    Code:
    bcdedit /store D:\boot\bcd /set {bootmgr} device boot
    Restart, and boot from the second disk. It should boot smoothly. Check
    Code:
    diskmgmt.msc
    for confirmation of GPT disk.


    NOTE : Tested in a VM, not yet tried on a real machine, which I will very soon. Hence I have not made it into a full tutorial. But realistically, it's not really going to fail unless something gets screwed up. Testers also very welcome! but rest assured, none of this is close to as dangerous as a Hybrid MBR. I'll make it into a tutorial once it's tested and no sudden issues are found

    I'll be looking for more elegant solutions very soon, such as mapping an image at boot time. But I wanted to tell (announce :) ) to you guys, so that I get some valuable feedback, and improvements, and expert gyaan.

    The original thread is here : Hack Bootmgr to boot Windows in BIOS to GPT - Windows 7 - reboot.pro . Credits to cdob and Sha0 from reboot.pro.
    Last edited by milindsmart; 23 Aug 2014 at 22:00. Reason: added image install, fixed short "basic principle"
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  2. Posts : 72
    Windows 7 Home Premium x64
    Thread Starter
       #2

    One caveat is, of course, that the Windows install/repair will refuse to automatically repair your installation of windows.. because it obviously doesn't know how to handle this "unsupported" configuration. Since it's not like there'll be a dedicated expert to "support" my computer when using supported configurations, I think it's worth the effort of manually working with it.

    There is a way however, for automatic repair to be used, for non-boot related problems : that is, winload.exe and other system files, not bootmgr. Use DUET to boot into the windows install disc. This should allow the installer to do all the repairs it can.
      My Computer


  3. Posts : 1
    Windows 7 Ultimate x64
       #3

    I have an efi computer, but the disk won't boot with efi, so I might have to do this. Since the computer is efi, is there a way to install using this method but then move the boot files over and boot without the external drive?
      My Computer

  4.    #4

    walkerneo said:
    I have an efi computer, but the disk won't boot with efi, so I might have to do this. Since the computer is efi, is there a way to install using this method but then move the boot files over and boot without the external drive?

    The solution which works for all others who don't want to waste any more time with EFI is to Bypass UEFI to Install WIn7.
      My Computer


  5. Posts : 72
    Windows 7 Home Premium x64
    Thread Starter
       #5

    Best bet is to find out why and try to fix your computer into booting with UEFI. Failing which, skipping UEFI might be easier.

    If you're keen on using GPT with windows without UEFI, and you don't want to use an additional disk, check out this answer, which is an improvement of the above method.
      My Computer


  6. Posts : 3
    Windows 7 Professional 64-bit
       #6

    So, just to clarify some things in your original post milindsmart...

    The C drive is your boot drive and the D drive you refer to later in the tutorial is your usb flash drive correct?

    I don't install anything on the flash drive prior to starting this process?

    I install Windows on the drive I want to convert to gpt in the tutorial too right?

    Also, the USB flash drive will hold the bootmgr for good or just temporarily? What size can I use? Is 128MB too small?

    This will work on 4TB drives as well? I eventually want to install 4 4TB drives in my computer, but for now I'll be installing it on a 3TB drive.
      My Computer


  7. Posts : 72
    Windows 7 Home Premium x64
    Thread Starter
       #7

    Sorry for the delay, didn't find time to reply properly.

    robb20 said:
    The C drive is your boot drive and the D drive you refer to later in the tutorial is your usb flash drive correct?
    Yes C is the boot drive (one with \Windows\ directory) and D is the system drive (one with \bootmgr and \BCD)

    robb20 said:
    I don't install anything on the flash drive prior to starting this process?
    Nothing required. In fact you just need to set up an MBR and VBR on one of the primary partitions in the flash drive. You can use the rest for data.

    robb20 said:
    I install Windows on the drive I want to convert to gpt in the tutorial too right?
    Yes. You can also go ahead and convert the drive, and then install Windows by imaging it onto the drive directly, using DISM or ImageX.

    robb20 said:
    Also, the USB flash drive will hold the bootmgr for good or just temporarily? What size can I use? Is 128MB too small?
    For good. Anything beyond 32MB is fine.

    robb20 said:
    This will work on 4TB drives as well? I eventually want to install 4 4TB drives in my computer, but for now I'll be installing it on a 3TB drive.
    Yes it'll work on any drive windows already can boot from.
      My Computer

  8.    #8

    What are comparative boot speeds with win7 system files throttled by USB controller?
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  9. Posts : 72
    Windows 7 Home Premium x64
    Thread Starter
       #9

    According to Wzyboy here and here, there is no visible impact on speed.
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  10. Posts : 3
    Windows 7 Professional 64-bit
       #10

    No worries milindsmart!

    Thanks for all the help! I just finished installing Windows on a 3TB GPT drive thanks to you! :)
      My Computer


 
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