Comment 46 for bug 1766945

TLDR: the Xubuntu 18.04.1 installer does not recognize an NTFS EFI partition. Then the process takes it course and fails.

Longer story. I recently staged two dual boot machines from scratch. The only difference between them was the EFI partition.

Case 1:
* Laptop came with Windows 10 pre-installed
* Resized Windows partition
* Booted with USB. Xubuntu installer offered choice of installing along existing Windows system and everything worked like a charm. KUDOS TO THE DEVELOPERS OF THE XUBUNTU INSTALLER.

Case 2:
* Restaged desktop that had a Windows 10 digital license on blank SSD
* Resized Windows partition
* Booted with USB. Xubuntu installer did not offer choice of installing along existing Windows system
* STOPPED. Compared the two cases and found the difference: the fresh Windows 10 install (using 1809 media) set the desktop up with an NTFS EFI partition (at least so says gparted).
* Rebooted with USB. In Xubuntu installer chose "Something else", selected the free space on the disk to create a new partition and let the installer go
* The installer failed when trying to install grub-efi

I will still make some attempts at fixing this, and will post here if I find something new.