I had a nvme disk with GPT partition table:
nvme0n1p1 fat32 Label=SYSTEM size=400MiB PartName=EFI system Partition Flag=Boot
nvme0n1p2 unformatted size=16MiB Microsoft Reserved Partition
nvme0n1p3 unformatted size=8MiB Flag=bios/grub
nvme0n1p4 unformatted size=378.87GiB
nvme0n1p5 ntfs Label= Win 10 size=97.66GiB
nvme0n1p4 was previously working with Neon.
I also have 3 other disks but I disconnected them for the moment (so I wouldn’t stuff them up)
First up went to install Kubuntu 25.04
The installer didn’t detect the windows installation. Did a manual disk partition, ext4 for the installation, “/” as the mount point, Kubuntu as its label.
All good selected nvme0n1p4 as the installation location and the installer chose nvme0n1p1 as the boot.
Went all the way through rebooted all good so far….
Modified /etc/default/grub to allow os_prober, then Update- grub
Odd it didn’t find the Win10 installation. Did a lot of online searching trying a few things (I wish all information on the web was dated)
Rebooted to the installation disk to check the partitions
All still the same except the kubuntu partition (nvme0n1p4) now had the boot flag set.
Unset the flag.
Now it wouldn’t boot at all (as expected). I could boot into win10 via the bios.
Pretty browned off by now, so as a last resort, did a reinstall, but chose replace an existing partition, so that the installer didn’t have to alter the partition tables. Did a reboot, modified /etc/default/grub to allow os_prober, then Update- grub.
It now found the win10 and added to the boot menu.
Success!
So I think this may be a bug but I don’t have enough knowledge to say where it is. Is it os_prober (which seems to be regarded as dodgy) or the installer or simply a permission thing