Reviving a dying hard disk
Sometimes, when a hard disk starts to die, it's possible to get a few days to a few months of use out of it. They all eventually die, though. You're only delaying the inevitable, and losing all the data in the process too. This should only be used if you absolutely need a hard disk to put in something but the only spare disk you have has a bunch of read errors.
Using whatever OS or tools available to you, zero out the entire disk, retrying on errors. All the bad sectors on your hard disk at this stage might still just be "soft" bad sectors, which are fixed by a write.
dd on Unix-y systems for this because it chokes on errors.
I've done this on three hard disks to success. But, as mentioned before, their lifespan is quite limited. You might think you have cheated death by not seeing any read errors for a few weeks, but once death has touched a hard disk, it will come.