Comment 52 for bug 1396379

This appears to be the problem I ran into with Ubuntu Desktop 20.10 as reported here:
https://askubuntu.com/questions/1321431/ubuntu-desktop-20-10-on-usb-flash-drive-boots-only-on-original-computer

If it is the same problem (and not a limitation with the latest [but old] ASUS N551VW BIOS) then please escalate this ABOVE CRITICAL.

I'm sure a lot more Windows 10 users would delight to have a bootable Ubuntu system on a USB Flash Drive (or an externally USB-connected SSD, e.g. in an ICY BOX) if the "Device for boot loader installation" set to /dev/sdb would really write something there. I also noticed a couple of error messages during booting - one was only a few seconds on the screen so I was not able to capture it. I will post what I have (in AskUbuntu) if it is of any assistance to you. My laptop works perfectly (error-free) with Windows 10.

Apparently the workaround is to remove Disk 0 (Windows 10 drive) from the laptop. I will give this a try though I doubt that the casual user would go so far as to unscrew the back cover of their laptop to remove their HD/SSD.