Ubuntu 20.04 crashes when it copies from a external disk large data

BIOS

ASUSTeK product: X455LAB v: 1.0

You have a very old BIOS, v: X455LAB.208 date: 08/03/2015.

The latest version is 302 from 6/4/2019, and can be downloaded here. Make sure to select X455LAB as your model #.

Note: Perform good data backups before upgrading the BIOS.

Note: Upgrading the BIOS may, or may not, solve your external disk copy problem.

Disk

You didn't state how large your external USB 3.0 disk is, but if it's 1TB or larger, you'll need to connect the AC power adapter to the external case, or use a powered USB hub. The USB 3.0 port normally doesn't have enough power to support external devices with high power needs.

SMART

Start the Disks application, select the desired HDD (check both internal and external disks), go to the "hamburger" icon, and select SMART Data & Tests. View the SMART Data window. Edit your question, and add the screenshot(s) if you wish me to review them for you.

memtest

Since you recently added RAM, go to https://www.memtest86.com/ and download/run their free memtest to test your memory. Get at least one complete pass of all the 4/4 tests to confirm good memory. This may take many hours to complete.