One of my backup drives on the NSLU2 started to fail (it won’t always spin up), so I nipped into Canada Computers and bought a 320GB replacement for a moderate number of $. Swapped the old drive out of the Vantec NexStar case, and — nothing.
Seems that the first-gen NexStars only support up to 250GB. Looks like I’m off to the computer store tomorrow.