Originally Posted by
oldfred
You have to have either both systems in BIOS mode or both in UEFI mode to dual boot. Windows only boots from gpt partitioned drives with UEFI, so you need to have Ubuntu in UEFI mode.
The install CD or flash drive has two modes to boot one UEFI and the other legacy/BIOS/AHCI or .. How you boot installer is how it will install. So you need to boot installer from UEFI menu in efi mode to do a UEFI install.
But Boot-Repair has a function to uninstall grub-pc (BIOS) and install grub-efi. It then updates fstab and runs the updates to get grub installed to efi partition. Grub2's os-prober also has a bug that finds the Windows, but creates the BIOS chain entry not an efi entry. Boot-Repair will also add a correct efi chain entry for Windows. So Boot liveCD in UEFI mode and run Boot-Repair.