how do I add a hard drive to the new Airport Time Capsule?

I just bought a 2 TB Airpot Time Capsule and it has a USB port on the back. I can plug in my USB drive (stand alone, powered via USB), and have it mount fine, but when I try to copy files to it, it gets ejected and a warning pops up.

So how do I use this USB port. Does the hard drive need to be formated in any fancy pants way?