Move macos user home folders on external drive
Having an old internal drive and a new external one which is a clone of it, I managed to move user accounts, from /Users to a new folder /Users/.legacy/ (for keeping them around just in case). I then created symbolic links of cloned user folders in /Users folder.

# cd /Users
# mkdir -p .legacy
# mv username .legacy
# ln -s /Volumes/New_drive/Users/username