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

I just went through a similar process and found that on Mojave, it's very difficult to mount an external drive on /Users or /home. It's best to use the standard /Volumes mount and change the home directory as described in TJ Luoma's comment above. Also, I used Carbon Copy Cloner to copy my home directory and I didn't have any permissions issues. More gory details are in my question/answer here and my gist.