ioSafe Solo USE External Hard Drive with Disaster Protection
Now here's an interesting product. ioSafe Solo External Hard Drive, Fireproof Waterproof Rugged USB in 500 Gb, 1 Tb or 1.5 Tb capacities.
Note however that the fire rating is only 1/2 hour at 1500 F. While this is probably quite adequate for most disasters if the fire is real bad this won't be sufficient. Or the proverbial tornado, etc. You will still need offsite backup.
That said I'd put this device in it's own closet with fire resistant drywall on all the walls and ceiling as well as a steel fire door. And a sprinkler.
My job description today is paranoid pessimist.