I see another trip to Tiger Direct in my future.
 I was willing to accept that the new 1TB disk drive had failed, since I've had that enclosure without problem for the last 10 months and disk drives sometimes fail out of the box. But I swapped it for the other 1TB drive I bought last night (same model) and have the same problem. Two drives? Unlikely. On a hunch I plugged the formatted 1TB drive into my external 3-in-1 adapter and it's showing up fine, which tells me yes, it's the enclosure. Great.