Question : Problem: How do I remotely access my hard drive from the Internet using Airport Extreme router with a PC?

I have a MAC Airport Extreme router with a hard disk plugged in the USB slot of the router.  How do I access the hard disk from the Internet (outside of my home network, LAN) using a PC not a MAC?

Answer : Problem: How do I remotely access my hard drive from the Internet using Airport Extreme router with a PC?

The short answer is NOT VERY EASILY.

The Airport Extreme router is designed to share the hard Drive with internal network users, NOT externally over the internet.

The only way to do it is to map the attached drive onto a PC as a network drive and then enable SSH or FTP on the PC.  You'll also need to open the relevant ports on the Airport Extreme to allow the traffic through.