We would recommend using dietpi-drive_manager to attach, mount external drives. Although, /etc/fstab is also fine.
If you want the Samba Server to point to the USB drive, and, you used dietpi-drive_manager to setup the USB drive, use dietpi-drive_manager or dietpi-software to move your DietPi user data, from SD to USB drive. Samba will then point to USB drive. https://dietpi.com/forum/t/dietpi-software-user-data/400/1
Or edit /etc/samba/smb.conf and change path = /mnt/dietpi_userdata accordingly.