Re: Bad sectors?
Bad sectors get locked out from being used because they can no longer do what they were designed to do. Chkdsk /r runs a scan and if bad sectors are found they are locked out. Plus repair bad system files it finds.
Bad sectors can come from normal wear on the platter, factory defects, abuse such as dropping the drive. If you're getting a lot of bad sectors on scans then it might be prudent to consider replacing the drive. RMA if under warranty.