I have been using
DirSync Pro (entirely free and open source) for many years and swear by it. What I love most about it is that it manages obsolete files and only backups files that are new or have been modified, so there is little overhead.
I back up my laptop's drive to an external USB drive roughly once a week. Important files, such as a ZIPed backup of my Calibre library, additionally live in cloud space (and on my tablet).