Currently I've many photos and videos made over the time and I don't want to miss them anymore. (BTW: It's about terabytes). To be sure in case of a burglary, fire or whatever I want to store it NOT at home. This leads me to the problem how do I keep in sync? Internet is not a solution since my bandwidth at home is very limited compared to TB's of data. On a single weekend it's not an uncommon case that I've to sync more than 8GB of new data. Since it's not at home I would feel much saver with encryption.
My best idea for the place is at work. I have an office, sharing with roommates I really trust. I thought it should be a place I visit regularly.
My best idea on how is putting a diff onto a portable hard-disk which is patched onto the backup system. (I think I cannot avoid to buy and setup a new system with encryption and RAID, etc.)
Unless you tell me the totally new app I would write a little script to do that diff. By the way, I would have Internet access on that box for diff-generation, but not for data transfer.