Dual Boot on Separate Hard Disk Drives

[Shadius]

According to the output, you do not have an EFI partition in your Windows drive, therefore it looks like you have Windows 10 in Legacy mode.

Can you confirm that you originally started with Windows 7 (or 8) and then upgraded in situ until you reached Windows 10?

No, I did not do an upgrade install. It was a new Windows 10 install.

[dragonfly41]

This is what I use with Windows 10 plus an old Ubuntu installation on /dev/sda
and Ubuntu 18.04 on SSD in /dev/sdb

rEFInd .. an alternative boot loader ..

If you can get into Ubuntu install rEFInd thus ..

https://www.rodsbooks.com/refind/installing.html

https://www.rodsbooks.com/refind/

sudo apt-add-repository ppa:rodsmith/refind
sudo apt-get update
sudo apt-get install refind

At the point of installing refind you will be prompted ..where?
and you point to your EFI partition ..
this can be the Windows EFI partition but if you are canny
for future reference you can have an EFI partition with your external Ubuntu.

Later you can read up how to customise refind.conf.

At bootup you should see reference to /EFI/refind/refind_x64.efi buried with other options and choose that
to skip through options such as Windows > Ubuntu and others which you can later suppress in refind.conf.
I leave them in..

[tea for one]

No, I did not do an upgrade install. It was a new Windows 10 install.

That's a surprise that you decided to install in Legacy mode (without an EFI partition) as shown in your post 14.

Anyway, you need to decide how you want to choose your OS when powering on your PC.

• Boot using the Boot Device options using the dedicated function key of your PC.
• Update grub in Ubuntu and follow oldfred's info in post 18.
• Use the suggestion from dragonfly41 in post 22.

My preference is still option 1 but I'm sure other users will support their opinions with valid evidence and experience.

[dragonfly41]

Should also point out the rarely documented option listed in post#2

"Generally easier to disconnect"

That is, Windows EFI boot flags can be temporarily disabled (through Gparted) or .. less practical .. cable to Windows drive disconnected.
You are left with only the external drive to boot. But this is a bit messy and requires EFI partition in external drive.
Some poster in an earlier thread some months back suggested a plug in switch which would isolate either disk (internal/external).
Overall I found from personal experience rEFInd to be easy to switch.

[P.S] Found the link ...

https://www.amazon.com/Device-Switch.../dp/B01MAX56SA

[Shadius]

That's a surprise that you decided to install in Legacy mode (without an EFI partition) as shown in your post 14.

Anyway, you need to decide how you want to choose your OS when powering on your PC.

• Boot using the Boot Device options using the dedicated function key of your PC.
• Update grub in Ubuntu and follow oldfred's info in post 18.
• Use the suggestion from dragonfly41 in post 22.

My preference is still option 1 but I'm sure other users will support their opinions with valid evidence and experience.

I'm not really sure how Windows 10 got installed in legacy mode as I left everything as default when I did the installation. It's strange to me.

I think I'd like to go with oldfred's option. I was used to the GRUB menu when I only had Ubuntu.

[Shadius]

Thank you for the informative reply - much appreciated.

Therefore, when selecting the OS, there is a distinct advantage to use UEFI/BIOS Boot Device Options rather than grub when the systems are on separate drives.

UEFI firmware is not affected by OS upgrades, albeit Windows or Ubuntu.

Ahh, I see.

With BIOS that was true. BIOS and operating system were totally separate. But with BIOS updates, it reset to defaults and you often had to redo your settings.

But with UEFI some settings are reset & some are remembered. And operating system can modify some UEFI settings. Windows typically syncs BCD with UEFI boot options and that is why you often have to add another setting to BCS for Ubuntu.
Other settings are like BIOS and are only changed if you do an UEFI update. But both Windows and now Ubuntu (fwupd) may do UEFI update changing some settings back to defaults and causing issues.

Thank you very much for this information. This changes things now.

[Shadius]

That's a surprise that you decided to install in Legacy mode (without an EFI partition) as shown in your post 14.

Anyway, you need to decide how you want to choose your OS when powering on your PC.

• Boot using the Boot Device options using the dedicated function key of your PC.
• Update grub in Ubuntu and follow oldfred's info in post 18.
• Use the suggestion from dragonfly41 in post 22.

My preference is still option 1 but I'm sure other users will support their opinions with valid evidence and experience.

Should Windows 10 be running in UEFI mode with an EFI partition? How would I go about correcting it?

[oldfred]

Supposedly you can convert to UEFI, some have said they have done it.
But Windows requires gpt partitioning for UEFI boot, and normal re-partitioning erases drive. Or you need good backups.
Windows also requires totally different set of partitions with UEFI/gpt, so often better just to re-install & restore your data.
If Ubuntu on gpt partitioned drive, you only have to reinstall the UEFI version of grub to convert. But again if MBR, probably easier to backup, re-install & restore from backups.

BIOS & UEFI Windows partitions, note system has totally different format & meaning between BIOS & UEFI
https://msdn.microsoft.com/en-us/lib...=vs.85%29.aspx &
https://docs.microsoft.com/en-us/win...Configurations
https://msdn.microsoft.com/en-us/lib...Configurations

Do not use on Windows drive. Better if drive is just a data drive.
Converting to or from GPT - must have good backups.
http://www.rodsbooks.com/gdisk/mbr2gpt.html