thesehands
You don't need to that via steam. You can do it with the filesystem itself.
For example, if your other disk is mounted /other/disk
, you can move it to the other drive and then symlink it back.
First shutdown steam. Then do:
sudo mv ~/.local/share/Steam /other/disk/.
ln -s /other/disk/Steam ~/.local/share/Steam
That should work unless you are using a flatpak or similar with a sandboxed library location.