Booting Ubuntu on the hard disk from USB

[Liam Proven]

I cannot get something to work which I used to be able to do fine.

I am wondering if I have forgotten how to do it, or if it no longer
works in Ubuntu 20.04.

With older versions, what you could do was:

Install Ubuntu to hard disk as normal. Let's say you are dual-booting
with Windows in /dev/sda1 and Ubuntu is in a logical partition,
/dev/sda5.

Something goes wrong or you break it and it won't boot.

What I used to do was put in a bootable Ubuntu USB, start it, and then
in the boot menu choose the "edit startup options" box.

I would remove the stuff about Casper and so on, so it said

[original boot stuff inc initrd] root=/dev/sda5 nosplash nomodeset

... and then press F10.

It would load the kernel and initrd from USB but then continue booting
from my installed OS.

Then I could install a new kernel or whatever, or reinstall GRUB, and fix my OS.

But I can't get this to work with 20.04. I always get the live system from USB.

What am I missing?

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