Just want to start off by saying how great BitTorrent Sync is and what promise it holds as a game changer for data backup and storage especially in combination with NAS drives. But this brings up the issue of data availability, requiring that the various devices holding a copy must be on to allow for the data to be shared with a new member of the data group. This might sound crazy, but a solution to availability would be to share your data with the entire world of BitTorrent Sync users. It follows from a paper written a while back on secure data storage in peer-to-peer systems. Supposedly one cannot trust the user base out there to store a full file of data, but one could encrypt a file and break it into pieces. The pieces could be shared redundantly improve the statistical likelihood of data availability. Each member of the community might provide a portion of their drive for data storage which is not theirs. This portion could be used as a fragment repository for random data members in the community. There are obviously other problems here. One could also use one's social network to store fragments. You might trust your friends more than a stranger on the Internet to store some of your fragments. Paper written here discusses a working prototype of the idea (Fragmentation-Replication-Scattering) here: http://kar.kent.ac.uk/14422/
