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I'd like to start by saying that i am fully aware that windows 7 is no longer supported, and i am also aware of the implications of running an outdated vulnerable operating system on my physical machine. Moving on to my problem. I currently have a drive with grub boot loader on it with Arch linux. The drive uses GPT. I needed to dual boot with windows 7, however i am only able to boot into the windows 7 installation by enabling CMS in my motherboard. I did read that i can hack the windows 7 iso to have true uefi support by patching it with efi files, vga drivers etc but sounds like too much pain.

I booted into the installation and got till the part where it asks me to chose a partition to install, all fine and well. At this point i expected the windows installer to point out that my drive is GPT and im booted in BIOS mode and therefore the installation cannot proceed without wiping my drive, however it has not pointed that out. From the looks of it i can simply select a partition (or create one) and proceed with the installation.

However, i did not proceed because i am not sure of what the result of doing such an installation is and therefore i am not prepared in case im left with a kaput machine that will not boot without further troubleshooting. So hopefully somebody that has done this before or knows about this can tell me what i should expect after the installation, and what tools i need to have on hand to mitigate any problems in booting.

Thanks.

Edit: It seems that even though you have to enable CSM in order to get into the actual installation of Windows 7, it is still in UEFI mode and you can install as normal. Windows will add its efi files to the already existing efi partition. Then you can use your bios uefi utility to select which efi file has to be prioritized in the boot process. In my case i set it to grub and then used os-prober to add the Windows boot loader to the grub menu. Note that you will need to have CSM enabled in order to boot into Windows 7 even after installation - if you disable CSM and attempt to boot, your machine will crash. Tried this on Windows 7 N ultimate SP1 x64 bit. I'm making this edit because the question is closed and hopefully this information might be able to help someone.

neo-devnull
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  • You cannot install Windows 7 while in BIOS (also known as Legacy or Compatibility Mode) on a GPT disk. It is impossible to do that. **Windows 7 fully supports UEFI mode which means GPT out of the box without any “hacks”** – Ramhound Jan 15 '21 at 12:44
  • @Ramhound Apparently not, could you please cite your source to the claim that Windows 7 works out of the box with true UEFI. i could not get them to boot into the setup unless it was with CSM enabled. I tried off of ventoy, off of a dvd disk, off of a flash drive that was dedicated to windows 7 and prepared with RUFUS from A windows machine - still no luck. Any information is appreciated – neo-devnull Jan 15 '21 at 12:47
  • UEFI existence predates Windows 7. Windows 7 doesn’t support Secure Boot but **all** Windows OEM devices are required to allow the user to disable it. Does not change the fact the Windows environment if booted Legacy mode cannot be installed on a GPT disk. If the disk is being listed it means your not in Legacy mode. If you were in legacy mode the disk wouldn’t be offered – Ramhound Jan 15 '21 at 12:55
  • [This](https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/boot-to-uefi-mode-or-legacy-bios-mode) article applies to Windows 7+ – Ramhound Jan 15 '21 at 12:57
  • @Ramhound Secure boot is turned off. Im pretty sure im in legacy mode because without being in legacy mode i wouldn't be able to get past the "Starting windows" screen of the installation. – neo-devnull Jan 15 '21 at 12:59
  • [This](https://superuser.com/questions/389971/how-to-move-an-existing-installation-of-window-7-64bit-to-uefi-from-legacy) also further confirms Windows 7 supports UEFI and this can be installed on GPT without any modifications. CMS typically doesn’t means a hybrid mode (which means it supports both MBR and GPT). – Ramhound Jan 15 '21 at 13:01
  • @neo-devnull: If you even reach the "Starting Windows" screen at all while in UEFI mode, that's already an indication that the source has at least some UEFI support -- e.g. if the DVD didn't have an EFI boot image, the EFI firmware wouldn't be able to start anything from it _at all_, it wouldn't even get to this part. (Though, of course, if you do this with a USB stick then you still cannot know whether the _original_ ISO had EFI support, or whether it was added by Rufus/Ventoy during the conversion...) – u1686_grawity Jan 15 '21 at 13:02
  • @Ramhound Not really, all the solutions in there are talking about CSM ENABLED. Either leaving it on and setting UEFI first or having windows 8/10 recovery to fix your boot problems(I have no idea how this is supposed to work), towards the end there is one solution which is clearly "hacky". – neo-devnull Jan 15 '21 at 13:30
  • @user1686 sure but thats pretty useless if you cant install the OS. – neo-devnull Jan 15 '21 at 13:31
  • Are you using a USB 2.0 or USB 3.0 device to boot into the Windows installation environment? – Ramhound Jan 15 '21 at 13:56
  • @Ramhound a usb 2.0 – neo-devnull Jan 15 '21 at 20:25

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