AFAIK the bad sectors are hidden from the OS as well. The bad sectors list is maintatined by the HDD, it is still not clear whether the bad sector list comprises of both the p-list(factory defects) and the g-list(growing list of sectors) or just the g-list. when the sector is marked bad , the HDD maps that bad sector to the spare sector(this is known as reallocaton count0, so if you see the SMART attriburte number 5 , it will give the reallocated sector count.