Does Scan Disk and Check Disk really help to fix the hard disk error?

"I don't think that they'll fix the errors, but they will find and mark those sectors that are damaged so that when won't be written to in the future. This should stop write errors (at least for a while).

If you are getting these sorts of error it could indicate that the hard drive is failing, so you might be better off investing in a new one and transferring your data."

"I have been trying to mirror my OS disk, but the process keeps stopping with a ""Disk I/O"" error. Eventually I found a Microsoft Support article that explains why:
http://support.microsoft.com/kb/325615
The article explains that CHKDSK and Scandisk will not remap bad sectors at a hardware level. They only mark bad sectors at a filesystem level. (The disk mirror process copies the hard disk sector for sector.) Therefore, you need a low-level error-checking utility that will remap bad sectors in the hard drive's own table. You can use SeaTools for DOS to do this. Another, proprietary, solution is SpinRite by Gibson Research.
I disagree with Chris' answer, but your question is also not clear. What kind of ""hard disk errors"" are you talking about? Bad sectors are somewhat like bad pixels: The manufacturer will allow disks with a certain number of them to pass inspection.
Remapping bad sectors with a low-level tool will stop error propagation, but new damaged sectors could occur if there is a physical problem with the drive."