Frosty

We have 2 hard drives, in different physical locations, connected only by the internet. Both drives have exactly the same data, copied manually, BitTorrent sync has never been used. We want to setup BitTorrent sync so as one of us works from a hard drive, any changes are mirrored to the other. Will BitTorrent sync recognize the files are the same, or will it try to re copy the entire 3TB of data? (Would take over 800 hours over the internet!) Many thanks