I need to backup one Seagate 3TB external drive to another. Dynamic backup would be great, if available.
Since the software is not mentioned, you can use another solution called EaSEUS ToDO backup. The software supports Dynamic backup, plus there is a tutorial available that you can use for this procedure.
You can download the application through the following link: http://www.easeus.com/backup-software/personal.html
Note: The free version is good enough.
Tutorial information: http://www.todo-backup.com/support/tutorial/backup-dynamic-disk-volume.htm