I had a 6 month old Seagate do this to me about 3 months ago. I discovered it when my FlexRAID array failed it's monthly Verify check. It almost looked like a bit flip but I don't know enough about that kind of thing to say whether that caused it.
Anyway, I just swapped it out of the array, did a low level format from the command prompt and stuck it back in there. It now reports just like the other 4 in there and I have had no problems. I check it about twice a week, but it seems to have complete cleared up.