Only seeing ONE of two external HD's plugged into Apple Airport Extreme
I've run into a similar problem using two flash drives, same size, same manufacturer. For whatever reason, the hub doesn't differentiate between the two. Try right-clicking on the drive that you can see and renaming it from "manufacturer xxxxx" or whatever it's called to something unique like "external drive A". Then shut down the entire setup (Mac, hub, Airport in that order) and restart everything (Airport, hub, Mac). That may help.