What about having a small device, say based on a BeagleBone board (or similar low power) hooked up to a USB drive and wifi. Then set it up in your car to power up for a while when the car is turned off. When it power up, it looks at the network that it can connect to -- if it is your home network, then rsyncs from your home backup server. If it is an alternate (say, office network, or another place you visit frequently), it can rsync to that server.

If someone marketed a turn-key solution like this (home backup appliance, plus vehicle-based transfer appliance), would this be a good alternative to online based services?

That's a clever idea.

I have been wishing for something like that for years. I'd love to have all my music sync'd to car automatically when I get home. Even better if it can take all my docs and whatnot with me. Security would be an issue; you're upping your exposure immensely by driving your data around.

Ouse a pair of mirrored drives (software raid1) for my main home storage, with a corn job rsyncing that to an external USB drive running encfs (which I swap out approximately weekly with a duplicate in my office drawer) - I could happily wifi sync that encrypted drive to one in my car without worrying about it if/when it goes missing. (and I've got a spare RaspberryPi and wifi adapters on the project box too… I can just see the hassle of getting a DIY power supply from the cars "12v" system to reliably power the RaspberryPi being more time wasting than I'd be prepared to invest in this though… Noisy automotive power systems are a pain in the ass…)

Take a look at one of those phone power pack batteries. These already have clean USB 5.0V output, and can charge while it is in use.

Problem solved: http://www.mini-box.com/DC-DC