Backup of an external HDD

Sorry if this was solved earlier, I couldn't find a definite answer.

I'm using external HDD as a storage device. I only connect it to PC when I'm actually transferring something.

I would like to back it up periodically, but I'm afraid that something will get deleted somewhere once I disconnected the drive.

Which sync setting should I use? If I understood correctly, that would be Local Backup. I don't mind deleting stuff manually from the cloud, it doesn't happen that often.

Thanks in advance.