How do I move a folder and contained files in an external volume to another external volume using Terminal
Assuming the mount points are correct

ditto /Volumes/Behemoth/A /Volumes/Galactus/ 


should work (you had an extra ~ on the source side).

In case you want to move the data (as the text in your question implies)

mv /Volumes/Behemoth/A /Volumes/Galactus/


will work as well (or you just rm -r /Volumes/Behemoth/A after you've successfully copied with ditto).