Let’s combine A and B: take a HDD drive, add some cheap intelligence and bring it online. That’s all. And to make it attractive for both, backup and host (the one who keeps the HDD on his side), just make it both ways.
So, Alice and Bob live in the same town: Berlin. They have similar internet connection: VDSL50 with 50MBit/s downlink and 10MBit/s uplink.
Both of them are setting up a syncosync box and after a few setup steps, they have secured their digital live in the most economic and most social way: they are storing their backups on each others sos boxes.
After some time, the systems are in sync. Now, if Alice’s house is on fire and she is not able to rescue anything her house, may be she needs access to all of her data very fast: scans of contracts, important mails etc.
So she takes a taxi to Bob’s place. He hands her out his syncosync box and – as long as Alice can remember the passwords of her backup, she can access the data with the speed of a local ethernet. If she has now bought a new laptop and a harddisk, she gets also an new syncosync box, syncs Bob’s box most preferable locally, and gives it back to Bob.