Installing Ubuntu 18.04 on external HDD

Question

I'm trying to install Ubuntu 18.04 on my Seagate 1 TB (external hard drive). I created one partition for my files leaving about 100 GB unallocated for Ubuntu's installation. I followed steps in this tutorial (in the link below) used to install Ubuntu

https://www.tecmint.com/ubuntu-14-04-installation-guide/

except that I set the boot loader to my external hard drive. Everything went so well but whenever I restart my computer and choose to boot from external HDD, I get unknown file system error

Update

Solutions suggested in the AskUbuntu community are all about fixing grub and it seems not working for me since it based on locating root partition, In my case I tried every partition listed all of them gave "error: unknown filesystem "

Here is the method i used

ls
ls (hd0,msdos6)/
set root=(hd0,msdos6)
ls /
set prefix=(hd0,msdos6)/boot/grub
insmod /boot/grub/linux.mod
normal

I spend too much time trying to install different distributions of Linux such as Centos but I got the same error. I think rather than fixing grub I need to know what causes this issue in the first place. Is there something wrong with installation, or something else? How do I find out?

Update 2

I tried Boot-Repair ISO to fix boot issue and report says the following:

The boot files of [Ubuntu 18.04.3 LTS] are far from the start of the disk. 
Your BIOS may not detect them. You may want to retry after creating a /boot 
partition (EXT4, >200MB, start of the disk). This can be performed via tools 
such as gParted. Then select this partition via the [Separate /boot 
partition:] option of [Boot Repair]

is that means i need to create /boot just before root partition or at the start of the entire hard disk

The last update

At last, Finally it works

Problem was in bios (can't detect boot files), boot-repair ISO report suggested adding /root partion at the start of hard disk

https://help.ubuntu.com/community/BootPartition

the most important remark as the help article says boot partition must be located inside the first 100 Gb of the disk

- Reduce one of the first partitions of the disk in order to create 1GB 
  (=1000MiB) of free space at the start of the disk where Ubuntu is 
  installed. This free space must be located inside the first 100GB of the 
  disk (its end must not be located at more than 100GB from the start of the 
  disk).

Thanks for all of you guys. I really appreciated your help, I hope AskUbuntu won't delete this question because as far i know it can't be a duplicate of this

How to install Ubuntu on portable external Hard Drive?

Answer 1

Welcome to Ubuntu!

You will find current and complete instructions at Ubuntu.com, much more useful than a third party article designed for a version which has reached its End of Public Support, as version 14.04 did in April of 2019. Things have changed for 18.04 LTS.

For one thing, you don't need to create a swap partition, since the modern kernel is fast enough so there's no longer a disadvantage in using a swap file instead. The swap file will be created automatically during installation of Ubuntu, so please don't bother with a swap partition.

The Pre-Installation Hardware steps are especially important. You did not specify your hardware platform, and whether it BIOS/firmware relies on Legacy Boot, whether Legacy Boot is still enabled, of if you have disabled that option and set for the preferred UEFI boot.

Please use UEFI if you can, which probably will require a complete erasure of that external drive and making an entirely new GPT partition table, unless that drive is already prepared with a GPT partition table. You can confirm the partition table type for all drives with this command ( thanks to https://unix.stackexchange.com/questions/120221/gpt-or-mbr-how-do-i-know ) to run from any Linux:

sudo fdisk -l

If you see Disklabel type: dos the drive is MBR-formatted, not GPT-formatted. If you are free to repartition that external drive with a GPT partition table, please back up everythng on that drive, verify the backup works, then do another backup and verify to different media, as disk partition operations are potentially destructive of data. Then, convert following this article.