On attempting to install Ubuntu alongside an existing Windows installation on the same hard disk, the Ubuntu installer created a mix of MBR and GPT partitioning scheme, and UEFI/BIOS don't find any operating system on the hard disk anymore.I received some helpful advice from @grawity_u1686 here to delete the Ubuntu partitions, convert the partition table from MBR to GPT by booting from a Windows boot stick and running MBR2GPT.exe. However the command mbr2gpt.exe /validate /disk:0 fails with the error "Disk layout validation failed for disk 0" even though I can'tspot any obvious reason for that (there are only 3 primary partitions, there isplenty of unallocated space and the Windows installer has version 10.0.19041.964, not sure about the installed Windows version though).
This is what the drive currently looks like in gparted:
Is there any chance to make the disk bootable again while preserving the Windows partitions and to integrate them into the boot options? I was thinking about saving a partition image with dd, reformat and repartition everything, install Ubuntu again and recover the Windows partitions from the image, but I don't know if the boot manager can then be configured by hand to find this additional boot option. Also, is there maybe an easier solution?
Happy to provide any additional information that might be helpful, any hints would be appreciated!