A few notes, for the googlers of the world.
I just bought a 1TB drive to store my growing media collection. I attached it to my Airport Extreme (wifi base station, for the non-fanboys), which was running version 7.4.2 (the latest.) It showed up quite nicely in the Finder and worked fine unless it was under any kind of load. This appears to be a common problem. The solution proposed in this thread was to downgrade the Airport Extreme to version 7.4.1. There's a tech-note that discusses how to downgrade as well. (short version, in the Airport Utility, hold the option key while selecting Check For Updates and you'll get a list of prior versions. Who knew?)
I did that earlier this evening and the problem went away.
Eagerly awaiting 7.4.3...