Comment 89 for bug 1396379

The issue is that the wrong partition is mounted to /target/boot/efi, right? grub-install doesn't mount that, ubiquity does.

Look at the attached screenshot from here (in the zip file) https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1591352. When that screenshot was taken grub has not even been installed yet, it was still copying the filesystem.

/dev/sdb is the intended target to install ubuntu, but /dev/sda2 (windows esp) is mounted to /target/boot/efi. grub (the actual bootloader) is installed to /dev/sdb, but when the /boot files are copied the efi directory ends up on the wrong partition.

For whatever reason this borks window's esp and sda isn't bootable anymore, and if sda isn't present then sdb isn't bootable because it's missing the efi directory.