Cannot install Ubuntu 20.04 LTS to an external HDD

Install Ubuntu from a Pre-built Image File that boots BIOS and UEFI

It looks like you are doing everything right. At this point it might be worth installing Ubuntu from an image file. The image file will turn tour disk into a clone of a working, external Full install system, that boots in both BIOS and UEFI modes. It is quick and simple and always works.

If working in Windows:

• Download Image File: https://phillw.net/isos/linux-tools/uefi-n-bios/dd_unb_ubuntu-20.04_15GB_2020-06-26.img.xz

• Download Rufus*: https://github.com/pbatard/rufus/releases/download/v3.11/rufus-3.11.exe

• Double click Rufus exe file.

• Select USB drive as Target drive in Rufus.

• Select above Image File in Rufus.

• Click Rufus start button.

• Wait for flashing to complete... Done.

• Password is "changeme", change it

The USB drive should boot on almost any modern X86-64 computer.

Thanks to sudodus for the image file.

In Windows it may be necessary to install 7Zip before proceeding. Rufus will use it when working with the .xz image: https://www.7-zip.org/a/7z1900-x64.exe

If working in Ubuntu: you can use mkusb, Disks or Etcher to flash the USB drive. P7zip may be needed to extract the image.