So, I just connected a WD external drive to my Airport Extreme's USB port. Although the drive appeared in Finder, I could not assign it to Time Machine until I searched the internet for instructions and connected to the server and entered my LAN address. Although it seems to be working fine, i'm curious why it is so cumbersome. I would have expected that Network preferences would see any drive connected to the airport extreme and allow me to click through to set it up. ETA -- Actually, I just noticed that although I did a full time machine back up yesterday with the drive hard-wired to my computer, now that it's connected to the network, it seems that Time Machine is starting from scratch again with a full back-up over the network. At 650 gb this seems like it could take days. Why is it doing this? It seems so poorly contrived.