I like to keep multiple backups of my files. Backups of backups if you will. I have a few terabytes of data sitting my external drives. In particular, the “main” drive contains files I work with everyday. The data on that drive is ten times more important to me than my MacBook Air. The truth of the matter is, simple backup solution like Time Machine (Mac only) is not available for external drives so I had to look for alternatives.
One of my favorite apps to keep two external drives in sync is FolderWatch. The app is extremely simple. You specify your source and destination hard drive/folder and it takes care of the rest. It’s like a “cheap RAID” minus complicated setup and OS.
If you were looking for something like this, you can find FolderWatch on the App Store. I am not affiliated with the developer or Apple. This is just a solution that worked for me.