Here are instructions on how to have Time Machine switch between two backup hard drives.
My recommendations for setting up a comprehensive backup system for your Mac include swapping between two external hard drives. While one hard drive is being used to backup your Mac the other drive is kept off-site in a secure location. Using multiple drives provides protection against fire, floods and burglars, which could affect both your Mac and the backup drive sitting next to it. Read More from “Setting Up Time Machine To Use Multiple Hard Drives”
