Originally posted here by protocool
disk defragmentation removes the bad_sectors of your drives. but i have this experience using disk defragmentation doesn't remove all the bad clusters in my drive which is suppose to be removed. but still, one way to remove bad_clusters in the drive is defrag. if it doesn't work try to re-format the drive.
I agree that defrag does give your HD a good workout. However, all that defrag does is to read and then write data back to your HD. This will work as it will avoid bad sectors, but the data you get written back could be damaged (depends on whether or not it read the original version of the data correctly).

Scandisk works by by writing data to every sector and reading back the data it has just written to check that the sector is OK. If you get an error from scandisk this indicates that some sectors on your HD do not work. This in turn indicates that your HD is probably on its last legs - one bad sector is perhaps acceptable. What will happen in the near future is that when you try and read data, you will get a corrupt result, causing all sorts of weird problems.

And I've seen a few of these weird problems