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…)
