Adjacent to that, allow iOS device backup on Macs to non-boot drives. Many people might have a relatively small SSD as their boot drive with additional storage on (mostly) external drives. However, backing up iOS devices to a Mac (via iTunes pre-Catalina or Finder in Catalina) automatically uses ~/Library/Application Support/MobileSync/Backup.
System Preferences>Users&Groups>"Username"-Rightclick User=Advanced Option>Home directory