GRUB2 loader is not being installed into the root directory during ubuntu installation in a n external hard disk

[Harsh]

I have a LENOVO legion Y520 laptop and have created the installation media for installing ubuntu on an external hard disk in UEFI boot mode. After partitioning using gparted wherein all partitions created were primary partitions and starting installation, the installation crashes where it is mentionedthat grub2 can not be installed into root directory.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubiquity 2.21.63.6 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 4.13.0-36.40~16.04.1-generic 4.13.13
Uname: Linux 4.13.0-36-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.15
Architecture: amd64
CasperVersion: 1.376.2
Date: Thu Jul 19 03:00:17 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/ubuntu.seed boot=casper only-ubiquity nomodeset ---
LiveMediaBuild: Ubuntu 16.04.4 LTS "Xenial Xerus" - Release amd64 (20180228)
ProcEnviron:
 LANGUAGE=en_SG.UTF-8
 PATH=(custom, no user)
 LANG=en_SG.UTF-8
 SHELL=/bin/bash
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)

[Phillip Susi (psusi)]

Your disk is currently partitioned to boot in bios mode, but you booted the installer in EFI mode. You either need to boot the installer in bios mode, or partition the disk using GPT and set up an EFI system partition to install in EFI mode. This will be done for you if you choose the "use entire disk" guided install option.