I've upgraded my HDD to 500 Scorpio Blue for 2 months now...had one problem, but it seemed to fix itself. When I had the problem, it was quite a nerve racking.
I was flying overseas, carrying my macbook, then decided to turn it on whilst in transit between flights. A big question mark appeared momentarily after i turned on the macbook. I tried a couple times, but that failed to work.
Then after a posting on this forum, I realized it might be the G-shock locking mechanism that was activated which is featured on WD drives. Not confirmed, but thought to be worth sharing just in case any of you have this problem. Just turn off your computer, wait a couple hours, then turn it back on.
I've ran all sorts of disk scans, and nil problems detected. So hopefully there's nothing bad in the long run. No firmware upgrade from WD. Looks like this might be a rare problem.