Your fstab shows a commented-out entry for /boot/efi. This is the EFI System Partition, which must be mounted in order for bootloader upgrades to work. You'll need to correct this to complete the upgrade.
** Changed in: shim-signed (Ubuntu) Status: New => Invalid -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1575813 Title: Dual boot does not work after upgrade to 16.04 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/shim-signed/+bug/1575813/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs