I cannot test this, but I think it's worth a try. Without logging into X, switch to a console CTRLALTF1. Log in, make a directory to store your current home directory, and then modify the account to move the current home directory.
sudo mkdir /mnt/tmp
sudo usermod -d /mnt/tmp/$USER -m $USER
Login again without X. Define your home partition in
sudo vim /etc/fstab
It might look like this:
/dev/sdb1 /home ext4 defaults 0 2. When correctly defined, mount it, make sure permissions are ok, modify the home directory again, and then move the files from the old home to the new home.
sudo mount /home
sudo chown -R $USER: /home/$USER
usermod -d /home/$USER $USER
Log in again with X.
sudo mv /mnt/tmp ~
sudo chown -R $USER: /home/$USER/tmp
Does this work as intended?
You can create a new directory, like
/users. Mount the disk inside users. Change permissions on
/users/$USER to match your username, and then change your home directory path with usermod.