Airport Extreme with USB Drive and TimeMachine

I have an airport extreme base station with a USB drive attached to it. My Mac sees it in the shared section of finder, my Ubuntu box sees it on the network and so does my Windows box. The USB drive is formatted with HFS+

However, TimeMachine does not see it as an option to back up to. What am I doing wrong? I am on OS 10.6.

Your help is greatly appreciated!

-C

one more note...

One more note.... when I plug the USB drive directly into the Mac, TimeMachine likes it.