Windows Vista displays a big scary dialog when the hard drive's on-board circuitry reports that the hardware is starting to fail. Yet if you go to the Disk Management snap-in, it reports that the drive is Healthy. What's up with that?
The Disk Management snap-in is interested in the logical structure of the drive. Is the partition table consistent? Is there enough information in the volume to allow the operating system to mount it? It doesn't know about the drive's physical condition. In other words, "As far as the Disk Management snap-in is concerned, the drive is healthy."
Similarly, your car's on-board GPS may tell you that you are on track for a 6pm arrival at your destination, unaware that you have an oil leak that is going to force you to the side of the road sooner or later. All the GPS cares about is that the car is travelling along the correct road.