How do I completely disable my dedicated/discrete (AMD/ATI) graphic card in Xubuntu 18.04 & 20.04 live non-persistent USB?

The graphic card is fried and I think that's what's causing the session to freeze every time after the computer warms up.

Things I've tried so far...

1. Adding radeon.modeset=0 to the line GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" in the file /etc/default/grub so that it reads:

   GRUB_CMDLINE_LINUX_DEFAULT="quiet splash radeon.modeset=0"

2. Adding radeon.modeset=0in cdrom/syslinux/txt.cfg as well as syslinux.cfg

3. Logging in "safe graphics".

Obviously, non of the above worked.

Appreciate any help to solve this issue. Thanks loads in advance!

UNetbootin & Ventoy (I can use any other tool).

BIOS : Insydeh20 | Rev. 3.5 | F.29

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