I have the new AirPort Extreme 802.11ac. I plugged in my 2TB Western Digital external hard drive into the AirPort Extreme USB port. I tried to find the mounted hard drive in my networks on Windows and the AirPort app on iPad and nothing comes up.
Make sure the hard drive is formatted correctly and working properly. Do this by plugging it into your computer first. If you plug it into a PC because you don't have a Mac and it works, then it's probably formatted NTFS. This is bad because Airport Extreme can't read NTFS, and PC's can't format HFS+. You could reformat it as FAT32 but you will waste a lot of space depending on the size of your files. Your best option would be to find a friend with a Mac or just go to the Apple store and format your hard drive as HFS+.
In simple words, if a Mac can copy files to the drive, then it should work fine with the Airport Extreme. If the hard drive is formatted factory default and the box doesn't say "Mac" somewhere on it, then it probably will not work with the Airport Extreme without being formatted by a Mac.