Time Machine is an easy-to-use backup system for macOS that helps prevent data loss. With the introduction of OS X Mountain Lion (10.8), Apple updated Time Machine to support multiple backup drives. The update provided a more robust backup solution by allowing you to designate multiple drives as backup destinations.

The main advantage of having multiple Time Machine drives is that one backup is often not enough. Redundant backups ensure that if something goes wrong with one backup, you have a second or third backup to pull your data from. It’s a failsafe.

It is not uncommon for professional organizations to have backup systems that create two local backups that are used in rotation. The first might be for even days; the second for odd days. The idea is simple: if one backup fails for some reason, the second backup is only one day older. The most you would lose is a day of work.

Many companies also maintain off-site backups. In the event of a fire, flood, or other disaster, the company won’t lose all of its data. The idea of off-site backups has been around long before cloud storage systems existed.