I "fixed" it from a live CD by manually downgrading the efibootmgr package to 0.5.4-7ubuntu1 (ie the one from 14.04 that was previously working) and rerunning "grub-install /dev/sda", "grub-install --recheck /dev/sda" and "update-grub" from a chroot.
I had exactly the same problem on my W540.
I think it's an efibootmgr issue: https:/ /github. com/vathpela/ efibootmgr/ issues/ 7
I "fixed" it from a live CD by manually downgrading the efibootmgr package to 0.5.4-7ubuntu1 (ie the one from 14.04 that was previously working) and rerunning "grub-install /dev/sda", "grub-install --recheck /dev/sda" and "update-grub" from a chroot.