Have a look at BitTorrent Sync: http://labs.bittorrent.com/experiments/sync.html
One advantage is that there's no third-party storage of data involved.
Thank you, will have a look.
But how big is that volume? Shouldn't then Dropbox always upload the whole volume, even if you just made one change in one file?
To secure my data, I just use BoxCryptor. It creates an encrypted volume within my Dropbox. It is free for non commercial use.