Password Protecting Partitions on an External Hard Drive
Solution 1:
Your best bet is probably TrueCrypt, a full-featured encryption program. Put your 'public' files directly onto the hard drive, and then put a TrueCrypt volume onto the drive. TrueCrypt doesn't require any installation, so you can include a copy of the software and run it on any Windows computer; I don't know about other platforms, but I'd assume that similar no-installation copies are available for other platforms.