Everybody should have off-site backup of their files. For most people, the biggest threat is fire, but here in California, the most likely disaster you will encounter is an earthquake. Only a small fraction of houses will burn down, but everybody will experience the big earthquake that is sure to come in the next few decades. Of course, fortunately only a modest number of houses will collapse, but many computers will be knocked off desks or have things fall on them.

To deal with this, I've been keeping a copy of my data in my car -- encrypted of course. I park in my driveway, so nothing will fall on the car in a quake, and only a very large fire would have risk of spreading to the car, though it's certainly possible.

The two other options are network backup and truly remote backup. Network backup is great, but doesn't work for people who have many terabytes of storage. I came back from my latest trip with 300gb of new photos, and that would take a very long time to upload if I wanted network storage. In addition, many TB of network storage is somewhat expensive. Truly remote storage is great, but the logistics of visiting it regularly, bringing back disks for update and then taking them back again is too much for household and small business backup. In fact, even being diligent about going down to the car to get out the disk and update is difficult.

A possible answer -- a wireless backup box stored in the car. Today, there are many low-cost linux based NAS boxes and they mostly run on 12 volts. So you could easily make a box that goes into the car, plugs into power (many cars now have 12v jacks in the trunk or other access to that power) and wakes up every so often to see if it is on the home wifi, and triggers a backup sync, ideally in the night.

Of course, it's not that simple:

• Wifi signal may not reach the trunk of a metal car, though fenders have less metal these days.
• In many cars, perhaps most, the 12v jacks are only on when the car is on, which is not when the car is in the driveway.
• A network drive that's encrypted by a key only found on the main machine is not so easy on linux

You could hook the box directly to the car's battery without too much work, but it's not as simple as plugging into a jack. You don't want to drain the battery, so you would need a box that sat in low power mode most of the time, waking up and pinging machines that might want to backup -- if they are on -- from time to time, and no longer doing this if voltage drops below a certain level.

Another idea would be to put a small battery in the box, good for a couple of hours of operation, and to charge that battery when the jack is hot (ie. when you go out driving.) Then the box can wake up at appointed times and do backups. If you have a giant backup, you could do it over multiple days, or go out and turn on the car. Or go get the box and bring it into the house for initial full-disk backup.

One of the nicer ways to backup is to use a tool that just syncs two filesystems. The problem is that the disk in the car must be well encrypted. You don't want somebody to steal it and get all your secrets. Some NAS boxes do disk encryption but you must enter the password on every boot. They only protect against the disk being taken while off. And that's no good for unattended backup. You want the data to be encrypted at the source, and over the wire, and on to the disk. That doesn't work if you are just mounting a remote disk via NFS or SMB and syncing to it. One option is ecryptfs, which encrypts inside a regular filesystem.

Some dedicated backup tools which back up the whole system to one big file will encrypt that file. Those will work but they don't provide the one nice thing about syncing, which is you can now access your filesystem if all you have is the backup NAS. That would actually be handy if you are out driving and you need a file from your home systems.

But these problems can all be solved to get the real goal -- automatic backup of huge filesystems (ie. photo and video collections) without the network but not in your house, in a secure way.