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I have a GPT formatted SSD Drive that I have moved from a rather new laptop to an older one that has an old BIOS.

The drive has multiple partitions including Linux that I don't want to remove and a lot of data that I don't want to lose. (The drive is big enough to host a Windows 10 installation if i ever need one, although I practically only use Linux.)

Therefore, I don't want to re-format the drive and change to a MBR structure.

On the other hand, changing or updating to UEFI is not an option on this machine.

It must be Windows 10 + old BIOS + GPT drive or nothing.

I am able to boot the Windows installation USB drive, but it gives an error saying it's a GPT drive.

Can I install Windows 10 under these conditions by any means?

Would Windows 10 be installable if the drive was MBR?

cipricus
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    MBR2GPT could be used, but it's convoluted (I thought it would also do GPT to MBR, but I don't think it does - a utility likely exists, not sure if Linux/Win). Ensure drive doesn't have more than four partitions (EFI boot on Windows requires three partitions - the same recommended setup for BIOS: Boot, OS, & WinRE), and you'll need a WinPE USB containing the downloaded mbr2gpt utility. MBR: Boot WinPE USB, use mbr2gpt to convert. | GPT: Find a utility for Linux or Win that allows converting without formatting – JW0914 Sep 29 '23 at 11:01
  • @JW0914 - Ensure the drive doesn't have more than four partitions - well, reformatting the drive is not an option here, and if it were, and if only 4 partitions were acceptable, I would simply re-structure it as MBR – cipricus Sep 29 '23 at 11:04
  • mbr2gpt doesn't format, all data remains intact. There has to be a utility to convert the opposite way, and if so, MBR only supports four primary partitions, which is why I mentioned to ensure the drive only has four. Either such a utility would error out with more than four, or data would likely be lost on partitions 5+. There's no quick solution here - another option would be to create simple script to WIM the OS and other data partitions from WinRE before disconnecting from a UEFI machine, use DiskPart to wipe, convert mbr and setup new partitions each time (SSD recommend) – JW0914 Sep 29 '23 at 11:08

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It must be Windows 10 + old BIOS + GPT drive or nothing

If so it won't be, period.

Unlike most Linux based OSes, Windows strictly requires 'msdos' (MBR) partitioning for BIOS mode (or Legacy/CSM under UEFI) and conversely it strictly requires GPT for UEFI mode.

This is apllicable to any Windows version supporting both modes - Windows 7 and up to Windows 10 - and obviously not applicable to Windows 11 that only supports UEFI mode and GPT.

ChanganAuto
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  • I can take the drive out, and connect it to a UEFI Mac laptop: could I install Windows by booting its USB on the Mac and installing it on the (then and there) external drive? – cipricus Sep 29 '23 at 08:33
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    Windows, generally speaking, can't be installed in external drives (there was an official WindowsToGo that has been deprecated due to very poor performance and there may be unofficial ways to achieve it) but even if it could it would be a moot if you intend to use it with the old BIOS machine. – ChanganAuto Sep 29 '23 at 08:36
  • Thanks. Could you please take a look at my other question? https://unix.stackexchange.com/q/757723/341192 – cipricus Sep 29 '23 at 08:39
  • @ChanganAuto Windows cane be installed on an external drive easily via Dism, however it's not recommended because a USB connection isn't stable in the way a hardware installed drive is - bump the USB connection on either end and it risks faulting the OS and losing any unsaved data. The Windows installer won't install to an external drive, but the install.<esd||wim> could be applied via an answer file, or an existing install can be captured and applied with Dism – JW0914 Sep 29 '23 at 10:32
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    @cipricus That's an interesting question and I'm almost for certain the Windows bootloader wouldn't even be detected once the BIOS tries to hand off to the Windows bootloader, as the boot process, both at the motherboard firmware level as well as at the Windows Bootloader, is entirely different between MBR and EFI boot. Something that could be done is MBR2GPT could be used, but the solutions is convoluted and I'll explain in a comment under your question – JW0914 Sep 29 '23 at 10:46
  • @JW0914 - funny enough, I already have a Windows on a portable drive (in fact an old MBR internal drive that I took out) and I don't care very much for its stability because I only need it for a minute to fix other drives ntfs errors, as per https://unix.stackexchange.com/questions/748595/how-to-fix-external-drive-in-linux-the-way-windows-does-automatically and even https://unix.stackexchange.com/questions/757732/can-wine-and-or-playonlinux-be-used-to-fix-ntfs-errors-the-way-windows-does. I boot it from a Macbook that otherwise runs Linux :) – cipricus Sep 29 '23 at 10:53
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I haven't tried it with Windows 10, but it is possible to boot Windows 7 on GPT + BIOS either from memdisk or through Clover. It should work for Windows 10 as well. There can be issues with hibernation and Windows updates related to booting.

See also the respective Windows 7 question for more information and wimboot route.

Abradoks
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    While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review – Destroy666 Jan 01 '24 at 01:56
  • Thanks for the tip, but I doubt I'll ever use Windows 7 - beside the fact that the question is very specific and has nothing to do with that scenario, and also that, after accepting the NO answer, I have re-formatted the drive to MBR and installed Windows 10 (which doesn't mean I ever use it: I have also reinstalled Linux etc). – cipricus Jan 02 '24 at 12:19
  • These methods work for Windows 10, I was not implying that you should install Windows 7. Even if you have already resolved your problem, it is better for other people with the same question to find some directions instead of just the incorrect answer about it being impossible. – Abradoks Jan 02 '24 at 20:35