What do bad sectors of hard disk mean?

A bad sector is a sector of the disk to which data cannot be written. This may be due to a number of reasons, but to your operating system all that matters is that it can no longer use that portion of the disk. If the disk has yet to be used, or is being reformatted, bad sectors are not really an issue. All recent operating systems map a drive's bad sectors and avoid them while formatting it for use.
Modern hard drives are manufactured with 'spare' sectors which are only used to replace space lost due to bad sectors. This allows a drive to automatically compensate for any slight manufacturing defects which may have resulted in the drive having bad sectors 'out of the box.' When formatting a drive, bad sectors are detected and these 'spare' sectors are used to replace them as necessary.

Where bad sectors become an issue is when previously usable sectors become damaged due to disk error. Any data on these sectors is in danger of being corrupted or destroyed. The operating system has no way of automatically compensating for this loss, potentially leading to data corruption on a larger scale and system instability if the affected data was crucial.
Bad sectors can be created by software errors, or slight errors in the read/write heads which cause the data laid down on that particular sector to be completely illegible, and thus appear as bad to the operating system. This type of 'soft' bad sector can generally be fixed by completely erasing the disk (by writing every sector over with a pattern of zeroes).
Bad sectors created by a mechanical error have a tendency to breed more bad sectors quickly. Since the margin for error inside a working disk is so slim, any surface damage or debris created by a mechanical failure or 'head-crash' will quickly cause more damage as it interferes with the read/write heads. When formatted, a drive with 'hard' bad sectors like these will use the spare sectors to as much as possible replace the existing bad sectors as detailed above. Of course, if the bad sectors are still spreading due to loose particles or surface errors in the disk, the disk will still be unstable. At this point, the only option is to salvage as much data as possible from the drive before it dies completely.
Fortunately there are ways to fix or work around bad sectors on a disk.

Method 1 of 2: Windows 7 and Windows XP

1. 
   1 .Close any open files on the affected disk drive. In order for the repair process to work, you must close any programs, applications, and files saved or stored on the faulty disk drive.
2. 
   2
   Open your Start menu. The Start menu button is located in the bottom-left corner of your Windows desktop and may appear in the form of the Windows logo if you use Windows 7.
3. 
   3
   Select "Computer" from the Start menu. If you use Windows XP, this option may read as "My Computer."
4. 
   4
   Open the properties menu for the affected disk drive. The "Computer" folder will display a list of all your disk drives on the computer.
   • Right-click on the affected disk drive and choose "Properties" from the floating menu that appears. The properties menu will then appear on your screen.
     
5. 
   5
   Click on the "Tools" tab. This tab will allow you to access the error-check and repair option.
6. 
   6
   Click on the "Check now" button. You will then be presented with options for checking and repairing existing problems with your disk drive.
7. 
   7
   Select the option for repairing bad sectors.
   • Place a check mark within the box next to "Scan for and attempt recovery of bad sectors."
     
8. 8
   Repair the bad sectors.
   • Click on the "Start" button to repair the bad sectors on the affected disk drive. This procedure may take several minutes, depending on the size of the disk drive you are repairing. It may also ask you if you would like to schedule a disk check on startup. When the repair process has finished, Windows will notify you that the bad sectors have been repaired.
     

Method 2 of 2: Macintosh OS X

1. 1
   Insert the Macintosh (Mac) installation disc into the disc drive. The Mac OS X installation disc was initially included with the contents of your computer.
2. 2
   Select the "Install Mac OS X" option from your desktop. After your computer reads the installation disc, the "Install" option will appear in the form of an icon.
3. 3
   Perform the installation process. An installation wizard will appear on your screen to walk you through each step of the process. You will be prompted to indicate the affected disk drive that contains the bad sectors, as well as the parts of the operating system you want re-installed.
• Click "Install" after following the prompts within the installation wizard to re-install the affected parts of your Mac OS X system.
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