How do I roll back to catalina ?

Hi,
I'm having a major issue with Big Sur Beta, every time I attach an EX. HD to either USB port ( MB Air ), the icon appears but as soon as I open it the laptop crashes.

I can't live with this, it also means I can't attach my TimeMachine back-up.

So how do i go back to Catalina

There is no point continuing with Big Sur until Apple fix this issue with USB ports
I also wanted to return to Catalina, but the App Store refuses to download older installers. Even with the Catalina installer, I couldn't create an installation USB key from the Terminal. He replied that he could not find the "createinstallmedia" command.

(sudo: /Applications/Install macOS Catalina.app/Contents/Resources/createinstallmedia: command not found)

We are prisoners of Big Sur.

Only solution found : to use macOS Recovery.
a) create a disk partition.
b) restart with the Shift-Option-Command-R combination, to install the original OS of the machine on the partition.
c) then upgrade to Catalina.

Be careful to install the OS on the partition and not to erase the whole disk!

How do I roll back to catalina ?
 
 
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