Okay, I normally don’t come straight out with a non-technical answer to something like “what’s the best way to backup all my files?”, except that there is a really really simple way to do this.
Backblaze is a backup-to-the-cloud service which costs $5 a month for unlimited backups. It deals with external hard drives, duplicate files, etc etc and requires zero configuration. Simply install and you’re done. Thoroughly recommended.