On Tue, 2009-05-26 at 13:59 +0100, Jeremy Sanders wrote:

Strangely, the Reallocated_Sector_Ct is still zero on the drive even though 
Current_Pending_Sector is now zero. This is a Samsung HD103UJ by the way.

That can be ok. It probably tried writing and then rereading and when
that worked it decided the sector didn't really have a 'hard' error
(like a physical defect on the platter) and thus didn't need to be
reallocated to a spare sector. You could generate an unreadable sector
during a write by the power failing or with excessive vibration.

That said, the drive firmware could also be broken