I know this topic has been brought up before and is in the sticky FAQ: "Can I use BitTorrent Sync to backup from an internal drive to an externally connected USB drive? No, BitTorrent Sync will not permit you to sync one folder location on your computer to another location on the same computer (i.e. you can't have two shared folders using the same "secret" on the same device)" BUT I have been constantly looking for this feature and it seems like no one supports it. Drive, Dropbox, Cubby, etc., none support it. The utility is so obvious! We sync for both accessing files and backing up files, and being able to back up to an external drive creates another simple point of backup redundancy as well as keeping that external drive always up to date for taking it to someone else's computer. I am a software developer and I don't understand the difficulty. The use case is even simpler than syncing over a network. The software stack should remain the same until the networking layer, and at that point supporting this seems trivial. At some point the software must identify end points that are out of sync and begin the syncing process. Instead of sending the file over the network, just do a copy/replace! Anyway sorry to rant a bit but I would LOVE this feature and it seems like no one is willing to acknowledge that it's useful or put it on their roadmap. Otherwise I think BT Sync seems very promising.