Mint/Win10 dual boot *without* UEFI

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Zalbor
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Mint/Win10 dual boot *without* UEFI

Post by Zalbor » Thu Oct 01, 2015 5:33 am

Hi everyone,

I tried to search for this question in the forum but all the threads I was able to find seem to be about UEFI, so they don't really apply here. If I've missed one that explains what I want to ask below, please let me know.

The computer I use came with Windows 7. It was before UEFI became common, so it uses a classic BIOS*. The disk uses a traditional msdos partition table, with an MBR, primary/extended/logical partitions and so on. I installed Mint on it in addition, and the MBR currently uses GRUB to chainload Win7 (if I want to use Windows).

* or at least it emulates one. I think there is a setting in the "BIOS" to use UEFI, but it's off and it was off to begin with.

If I were to trigger the upgrade to Windows 10:

1) Would it overwrite the MBR? (Other threads confirm that it does overwrite the bootloader if you use UEFI, but not what happens if you don't.)
2) Would it keep overwriting the MBR after upgrades? (Again, other threads say that happens with a UEFI bootloader.)
3) If and only if the answer to (2) is "yes" (1 isn't a problem as I can reinstall grub), what's the best way to stop it? Suggestions I've seen are to make the Windows bootloader "chainload" GRUB, which implies it's impossible to stop (2), but again that was for UEFI.

Edited to add

4) Can GRUB chainload Windows 10 correctly?

Thanks for the information.

mintybits
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Re: Mint/Win10 dual boot *without* UEFI

Post by mintybits » Thu Oct 01, 2015 6:14 am

You don't need UEFI at all. Just upgrade to W10.
Grub will chainload W10 no problem.
I can't recall whether the MBR is affected but even if it is, reinstalling Grub is easy using your live Mint DVD.

Zalbor
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Re: Mint/Win10 dual boot *without* UEFI

Post by Zalbor » Thu Oct 01, 2015 6:20 am

And if I upgrade and then restore GRUB, is Win10 not going to overwrite it again when some updates are applied, as I've heard happens with UEFI?

Neophyte
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Re: Mint/Win10 dual boot *without* UEFI

Post by Neophyte » Thu Oct 01, 2015 7:18 am

Zalbor wrote:And if I upgrade and then restore GRUB, is Win10 not going to overwrite it again when some updates are applied, as I've heard happens with UEFI?
It should only override it when you upgrade to an entirely new OS (for example, if you reinstalled Windows 7) or when you reinstall Windows 10 from scratch.

Zalbor
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Re: Mint/Win10 dual boot *without* UEFI

Post by Zalbor » Thu Oct 01, 2015 7:27 am

That's good, thanks for the information both of you.

I'm still not planning to upgrade any time soon but it's good to have an idea of what to expect.

srs5694
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Re: Mint/Win10 dual boot *without* UEFI

Post by srs5694 » Sun Oct 04, 2015 10:05 pm

Zalbor wrote:And if I upgrade and then restore GRUB, is Win10 not going to overwrite it again when some updates are applied, as I've heard happens with UEFI?
Under UEFI, the boot loader does not reside in the MBR; instead, boot loaders are stored as files on the EFI System Partition (ESP) and the EFI stores an order of preference for which boot loader to use in NVRAM. Windows does not, to the best of my knowledge, overwrite GRUB in normal operation. (Depending on installation options, it might erase the entire ESP -- but this shouldn't happen in an upgrade from Windows 7 to Windows 10.) What Windows does sometimes do is to change the boot order so that the Windows boot loader takes priority over GRUB. This is easily changed with the bcdedit command in an Administrator Command Prompt window:

Code: Select all

bcdedit /set {bootmgr} path \EFI\ubuntu\shimx64.efi
Alternatively, the third-party EasyUEFI can do the job with a point-and-click user interface. In fact, EasyUEFI is likely the better option under Windows 10, since opening an Administrator Command Prompt window seems to be harder under Windows 10 than under previous versions of Windows.

Of course, none of this is relevant if you continue to boot both your OSes in BIOS/CSM/legacy mode, which is a perfectly reasonable thing to do in your situation. I just want to clarify this issue because there's a lot of EFI misinformation floating around out there, and I don't want this thread to reinforce this particular piece of misinformation.

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