My six years old QNAP TS-210 was key solution for my most important data backup back in 2012. That's the moment, when I first time heard about Unison (webpage, Wikipedia). Its idea (a free, self-hosted alternative to Dropbox, limited in size only by free space on your device) caught my heart and I immediately felt in love with it.
Unfortunately, after struggling with it for over a few months I finally failed getting it back to QNAP and turned into some alternative solutions (Bittorrent Sync, Dropbox, OneDrive). However, someone did finally find a solution and answered my four years old post. This obligates me to share this knowledge. Have a good reading.