All hard drives leave the factory with bad sectors; these are mapped in the drive’s firmware in what is called the P-List.
Hard drives develop bad sectors over their operational life time. Bad sectors can be the result of a tiny manufacturing flaw, or media degradation, or they can be caused by the read/write heads coming into contact with the media or platter surface itself.
In the former two scenarios, hard drives have the ability to “re-map” the bad sector, marking it as such, moving the data to spare sectors on the hard drive, and ensuring that that sector is no longer used. These bad sectors are mapped in the G-List. If a drive develops too many bad sectors, it will run out of space to store these, and the drive will effectively start failing.
