manual airport extreme base station

We are confused.

You ask.......How do I setup & configure my AirPort Express for an external harddrive.

Yet.....the title of your post states.....manual airport extreme base station

So, we don't know which AirPort you might have.

The AirPort Express does not support a hard drive, so you cannot use the USB port for anything other than a USB printer.

If you have an AirPort Extreme, you can set up a hard drive to work (quite slowly) at the USB port of the AirPort Extreme. Post back for more details on this if you really do have an AirPort Extreme.

An Airport Express's USB port may only be used for a printer: Cables you can connect to your AirPort base station - Apple Support.

AirPort Express 802.11n (2nd Generation) - Technical Specifications