I am sorry. I think I confused everyone. This is what I actually meant to say..

Usually a hard disk keeps spare sectors that are not visible to the user. Whenever the disk encounters a bad block the disk internally replaces these bad blocks from the spare sectors. This operation is completely translucent to the user. There seems to be a specific number of spare sectors on any hard disk that is used for recovering from bad blocks. But once the disk runs out of spare sectors in the process of replacing bad blocks, all future bad blocks will become medium errors (unrecoverable). If the number of available spare sectors can be found then the user can be informed much earlier that its time to backup this disk as it would soon become faulty.