install 20.4 alongside win 10 failed with 'grub-install/dev/sda' failed

[oldfred]

How you boot install media UEFI or BIOS is then how it installs for both Windows & Ubuntu.
Windows only installs in UEFI mode to gpt partitioned drive. If it sees MBR it will not install. And it needs unallocated space.
Ubuntu does let you install in UEFI mode to MBR drive, and really should not. It only creates gpt if drive is blank.

You can convert in place with gdisk, but Windows has major differences in partition requirements.
Converting to or from GPT - must have good backups.
http://www.rodsbooks.com/gdisk/mbr2gpt.html

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

[cimh]

Oldfred and others thanks again for all your help.

I have done a clean install windows 10 (knowing that it would force my hdd into GPT and UEFI. I let windows format 1/2 the HDD. leaving the other 1/2 for Ubuntu.
I made a usb of the ubuntu iso using rufus which lets the usb to be formatted to gpt and efi.
When installing Ubuntu it identified windows and I clicked 'other' to format the remaining part of the HDD as an ext4 partion mounted as \ for ubuntu - No problems

It all works except that the PC automatically boots to windows, rather than coming up with the ubuntu grub which would also offer the choice of OSs to launch.

So looking at the efi section I can see
efi\ms\Boot\bootmgfw.efi
efi\ubuntu\shimx64.efi
efi\ubuntu\grub64.efi

Some forums suggest that the following windows command will point the boot from the windows boot manager to ubuntu

bcdedit /set {bootmgr} path \EFI\ubuntu\shimx64.efi

But some other comments suggest that this might not be a permanent fix - so I think I'll play safe for now - the PC can automatically boot to windows - which will be great for my wife and i I want linux I can press F11 during boot and then select ubuntu.

So I guess my issue is solved thanks to everyone's help

[CelticWarrior]

Have you already checked the UEFI settings > Boot?

You should because probably all you need to do is to change from "WIndows bootloader manager" to "Ubuntu". This is because the Ubuntu installation not always change this setting but it can be done manually.

[cimh]

CelticWarrior THANK YOU again. In my MSI legacy startup it was obvious how to change boot priorities. But in UEFI all I could see was a list of 'fixed boot priorities' which I couldn't change and in which ubuntu did not appear anyway - so I thought that was that. However after your comment I can see lower down the page there is a link to "UEFI HDD BBS priorities" This shows a list of the 3 efi boots on the hard drive and this section lets me direct the boot to ubuntu rather than windows. So it all makes sense and the dual boot now works perfectly.

Many thanks - I have learnt something - I appreciate your patience!

cimh