How to connect to an external harddisk, which is connected to AirPort Extreme, vie iPad/iPhone/Windows Computer?

You might want to take a look at an App like FileBrowser

Stratospherix - Products - FileBrowser - Overview

Check with FileBrowser support on that. FileBrowser works fine for me to access files on computers, external and network drives from the iPad.

Are you saying that you cannot find the hard disk using FileBrower, or you cannot find the hard disk at all?

Make sure that the hard drive is formatted in FAT32 to work with Windows. Windows NTFS or exFAT formats will not work with the AirPort Extreme.

http://support.apple.com/kb/HT2426

This Apple support document will help you find the hard drive if you cannot see it in Windows Explorer:

http://support.apple.com/kb/HT1331

I am sorry,but I do not have a PC available at this time to test, since I am traveling. Perhaps another user can help with details, or a Windows support forum will have details on this.

If you have a Mac computer there, you can open Macintosh HD > Applications > Utilities > Disk Utility and format the drive correctly by connecting it directly to your Mac. Once it is formatted, you can plug it into the AirPort Extreme.

The documentation that came with your hard drive should also have information about how to format the drive for FAT32.