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I have installed 2 OSes in different two different disks.

  • SSD (250GB) installed Windows 10
  • HDD (500GB) installed Ubuntu 20 (Mostly used)

Windows 10 seems to be installed with legacy mode and Ubuntu 20 in uefi mode

Now when I start my computer the default OS is Ubuntu without showing grub options. It directly boot up in the Ubuntu.

Now What I want is that I would like to have Grub option before any OS boots up.(An option to be select to login in desired OS).

Exploring on the internet seems that both OSes have to be installed in same BIOS mode (Legacy or either uefi). I didn't find any solution for the my current situation.

So I would like to ask you that Can I keep both OSes without reinstalling them and choose which to use in grub options?

I have already two bootable USBs with Ubuntu and windows 10.

Thanks

Edit: I have tried Boot-Repair when I run the option "Repair boot", It shows me the error message: "LegacyWindows Detcted. Please enable bios-compatibility/csm/legacy mode in your uefi firmware, and use this software from a live-cd (or live-USB)".

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    I ended up with a similar situation, I press F12 every time I boot in order to select the Disk I want. I am not willing to risk either Windows or Ubuntu so I live with it. – C.S.Cameron Aug 10 '21 at 12:23
  • You can convert your Windows installation to EFI mode with little trouble. I had answered a similar question this way and it was successful. Let me add a link to the answer. – galexite Aug 10 '21 at 12:24
  • Here you are: https://askubuntu.com/a/1355459/287379 – galexite Aug 10 '21 at 12:26
  • @galexite I am almost convinced enough to try this with a Ubuntu full install pendrive, thanks. (my Windows is UEFI and Ubuntu is BIOS). – C.S.Cameron Aug 10 '21 at 12:43
  • @C.S.Cameron, this procedure will not work on a non-Windows disk, because it will create a Windows Boot Manager installation on the target disk, not a GRUB one. – galexite Aug 10 '21 at 12:47
  • If it’s just a pendrive, it’s better to just use Rufus to recreate the disk as UEFI (Non CSM). – galexite Aug 10 '21 at 12:48
  • No, I try risky things on a pendrive before trying them on my internal drives. So if I add a EFI partition and UEFI bootloader, or install GRUB in UEFI mode to the Ubuntu disk, will this method not work? I've not tried retrofitting a drive BIOS/UEFI before but I think it should work. – C.S.Cameron Aug 10 '21 at 13:18
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    I think the original advice by @C.S.Cameron is good: "I press F12 every time I boot in order to select the Disk I want. I am not willing to risk either Windows or Ubuntu so I live with it." -- But if you have a good backup and are willing to try and maybe have to restore your opering system(s), please go ahead, and remember to report the result here, whatever it it (good or bad) :-P – sudodus Aug 10 '21 at 13:29

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