Should backup discs be off during OS a update?

[Jerrybobbabe]

On an iMac should I turn off backup discs during upgrade to Big Sur from Mojave

 

[Apple_Robert]

I see no reason to turn off your backup program during an update to Big Sur. If there is a problem, you should still be able to downgrade back to Mojave.

 

[gilby101]

On an iMac should I turn off backup discs during upgrade to Big Sur from Mojave

I would disable any automatic backups until you are happy with the upgrade. You don't say what backup app you use. If Time Machine: TM has major changes with Big Sur. I recommend keeping your old TM device untouched and starting Big Sur TM to a new disk.

 

[Fishrrman]

I would both turn them off, AND disconnect them.
That way, if something goes wrong... they remain "untouched".