A bad sector on the hard drive represents a part of the drive on which data cannot be read or written, ie it is malfunctioning. The drive may have bad sectors due to a number of reasons. All current operating systems make a map of bad sectors in order to avoid them when performing any process.
Modern hard drives are made with “spare” sectors that are only used to replace space lost due to bad sectors. This allows a drive to compensate for any minor manufacturing defects that may have caused the drive to have bad sectors. Formatting a hard drive detects bad sectors, replacing them with “spare” sectors as needed.
Hard drive bad sectors become a problem when previously usable sectors become damaged due to physical disk failure. Any data stored in these sectors is in danger of being damaged or destroyed. The operating system has no way to automatically compensate for this loss, which can lead to data corruption.
Bad sectors can be created by software errors or interruptions during the read/write process, which makes data located in a particular sector unreadable or unintelligible to humans. Bad sectors can usually be fixed by completely erasing the drive and writing all sectors over again with a pattern of zeros.
Bad sectors created by a mechanical error have a tendency to expand rapidly, creating more bad sectors. Since the margin of error within a working disk is very small, any surface damage caused by mechanical failure will quickly cause more damage to appear, as the read and write process is interfered with.
It’s important to be proactive and periodically back up your most relevant data, especially if you discover that your drive has errors.