We have discussed the difficulty of dual booting OS installed in UEFI mode. I've been trying to find a way of booting IFL from the HD instead of using it from a USB flash drive as I like doing automatic restores without boot media. I tried EasyUEFI earlier this year. It was unstable and you never knew which OS would boot next. Ver 2.7 was released a few weeks ago and it's stable. You can set IFL for a One-time boot and you can control what IFL does by copying a script to the IFL partition prior to the One-time boot. After the One-time boot, Win10 boots.

I have an IFL partition on HD2 along with a large data partition and about five different WinPE partitions, all doing One-time boots when requested. Neither IFL or the WinPE partitions has a drive letter. In view of the number of primary partitions I needed, this 500 GB disk had to be converted to GPT.

I'm doing this on my test computer. My main computer has OS installed in MBR mode, which I prefer.

I have an entire drive image of Ubuntu which had been installed in UEFI mode. ESP, Ubuntu and Swap partitions. HD1 contained a single data partition. The partition was resized smaller and the disk converted to GPT. The three Ubuntu partitions were restored to the newly created Free Space on HD1.

EasyUEFI can do One-time boots of Ubuntu on HD1.