Attaching External HD to Airport - Is It Secure?
Disks connected to an Airport router aren't, by default, accessible from the Internet. This is for a few reasons:


Your ISP is almost certainly running firewalls that would make it hard to do this anyway.
By default, disks are only accessible from your local network.


That said, it's possible to make a disk accessible from the Internet if you want to - see this post on the Apple support forums. If you don't specifically take those steps, the disk won't be publicly accessible.