How often would you backup records to an external storage device?

In your Risk Assessment, you'll need to define how many records you can afford to lose (i.e. if your main disk farm goes down and you haven't backed up) against the actual likelihood of such a crash. Most hospitals are pretty content with a daily backup to an offsite location, but some of the bigger facilities prefer a more frequent backup. Don't forget to factor disk mirroring into this equation.