Originally Posted by nihil
Not only that, It requires an understanding of what the tools you are going to use will actually try to do.
Chkdsk is an intrusive, or active tool. It will run against your live production system and make irreversible changes to its contents.
Others, such as unstoppable copier and the HDD manufacturer's diagnostics are passive and non-intrusive. That is, they don't try to make changes to your production environment. The best ones run outside of the target environment and simply try to read its contents.
My preference is to use the passive, external, tools first to recover the data and attempt to determine if there is a hardware problem with the drive. Then try to repair the production system with other tools like chkdsk, fixmbr, partition recovery and the like, if there does not seem to be a physical HDD problem