Airport Extreme 802.11ac Can't See USB HDD
If the HDD is a 2.5' (laptop drive) that draws power from nothing else but the USB port it will never work with a USB Hub. They all share the same power source on the Airport that has a limit of 500mA. That would work with the hdd directly in the Airport but connecting an unpowered USB Hub draws around 80-100mA and the hdd is left with 400mA to start... which is too low.

I can only imagine, but i think you are using this setup to connect more than one drive to the Airport and having multiple disks shared by it. That's ok but...

For this to work you need a special USB hub known as a Powered USB hub that you can connect to the wall socket separately. This separate power adapter draws 2-2.5A of power from the wall socket to power each USB Hub port with its own 500mA thus being able to give enough current to the hdd to operate correctly.

Check this out. Notice the wall socket power adaptor.
http://www.ebay.co.uk/bhp/powered-usb-hub
Also try to get a decent brand so it lasts... and doesn't cause a fire or fry your hdd in the long run.