You have a couple different types of appeal you can make. They target different areas and you'll have to choose the best appeal for each boss.
1. Vanity - They're the boss. In theory they have access the most important data. They're also your most important customers and it might be a good thing to point out how uncomfortable you are with providing better service to underlings than you're able to give to the boss.
2. Safety - Much like car wrecks, computer security problems don't just happen to people who are careless, they also happen to people who get caught up in the wreck. Basically just a reminder that malware and zero day vulnerabilities can nail them even if they don't do anything wrong.
3. Convenience - When was their last backup? Outline what they can expect if their hard drive bombs.
4. "Platinum Plan" - S/he's the boss. Give them a USB drive and script up a regular backup. Or do it over the network. Kind of related to the Vanity entry.