What causes 'bad sectors' on a hard drive?

WD Internal Drives Desktop & Mobile Drives
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There are several causes of bad sectors on hard drives. Bad sectors may be a result of regular everyday use for a substantial amount of time. The problem of damaged sectors can also be present in new out-of-the-box hard disks. Physical damage or shock can even lead the hard drive to have bad sectors. If the hard disk experiences a strong jerk, the platter can be damaged, which might in turn create bad sectors. Another reason for damaged sectors is excessive generation of heat in the PC and improper ventilation settings.