Centered Systems Second Copy is the solution I use to keep files synchronized between a backup drive and my primary file folders. Second Copy, isn’t free and I realize not everyone wants to pay for file protection, especially with so many cloud backup solutions these days. A less versatile free option is Easy2Sync. The app is billed as a way to keep files synchronized between two computers, but it works just as easily to sync between two drives. A scheduling mechanism automates the comparison and transfer of files. The disadvantage of the freeware version of Easy2Sync is it won’t synchronize subfolders, only top level files in a folder. If you have a subfolder, it requires a separate configuration. Not a showstopper, but certainly less than ideal. Upgrading to a registered version removes this limitation, but if you plan to pay, Second Copy is a better choice.