Any recommendation on how to find out what’s eating up the space?
This will show the larges files on your system
du -a / | sort -n -r | head -n 20
If I move the rootFS to the external existing drive I have, is it formatted during the process?
yes usually the drive should be format before moving RootFS. Some instruction how to move RootFS Services Failing after Swapping Backup Drives - #6 by Joulinar
I can’t read the output quick enough before it goes back to the drive manager.
if you are connected via SSH, just scroll up on your SSH client.