Linux Mint: Move user's home directory to new drive #

Use Disks to format (e.g., ext4) new disk and set to automount (gear icon → Edit Mount Options... → disable "User Session Defaults" → enable "Mount at system startup" → configure Mount Point as desired, e.g., /newhome)
Boot into recovery mode (via Shift for BIOS systems or Esc for UEFI systems^* → Advanced options for Linux Mint... → Linux Mint...(recovery mode)) and select root
mount -o rw,remount /
usermod -d /newhome/bob -m bob

Notes

^*If you find yourself at a grub> prompt, type normal → Enter → Esc.
On one test system, had to reenable autologin afterwards:
1. add bob to nopasswdlogin group: Users and Groups → bob → click list of Groups → check nopasswdlogin → OK
2. change the auto login delay from 0 to 2: Login Window → Users → Automatic login → Delay before connection (in seconds) (as Dave B notes, this should be renamed to something more like "Delay before auto login (in seconds)"
Rather than moving the entire home directory, individual folders like Desktop and Downloads can be redirected by editing ~/.config/user-dirs.dirs then running xdg-user-dirs-update.
More complicated and error-prone methods top the search results; they may come in handy if Disks or usermod is not available:

How to automount hard disks on boot in Ubuntu
Ubuntu Recovery Mode - R/W File System
Adrian Ho's answer, which points out some advantages of this method over more manual processes, including "No typos possible - the home directory and /etc/passwd are guaranteed to remain in sync." and "In addition to the normal permissions, usermod also automatically preserves any ACLs (access control lists) and extended attributes."
Sukrit Gupta's comment on Command to change the default home directory of a user
A Complete Guide to Usage of 'usermod' command – 15 Practical Examples with Screenshots

/nix | Jan 28, 2021