This process assumes:
Problem: You have files stored on a DVD, when you try and copy them, you get errors, "unable to read file", "bad sector" or similar. Other DVDs work fine. Often, when you try and copy, the copy runs fast at first, then runs slower and slower before aborting with the error message. You find that smaller files are easier to copy.
Cause: This behaviour is due to bad media. The file copies slower and slower as the number of bad sectors on the disk increases, to the point where the drive can no longer read the disk. When this point is reached an error is generated. Prior to this point, the drive is using on-board error correction techniques to recover from errors, this causes it to run slowly. Large files are hard to copy because there is usually at least one bad sector in them.
Solution: Media Doctor has Bad Sector Mapping function, and a Patch File function. Used with two copies of a dead DVD, these functions can be used to recover the data. Use this process:
Sectors to replace the bad sectors listed in the BSM file will then be copied from the second copy of the DVD, and saved into the SPK file. Once the sectors have been copied, which may take some time (especially if the second copy of the DVD is also damaged), Media Doctor says "Operation Complete", but doesn't give a button to Continue, so we have to close and reopen it to continue.
Sectors to replace the bad sectors listed in the BSM file will then be copied from the SPK file, and saved into the copy of the recovered file on your hard disk. Once the sectors have been copied, which is very quick (unless the recovered file had many bad sectors), Media Doctor says "Operation Complete", but doesn't give a button to Continue, so we have to close and reopen it to continue.
Once the patch file has been applied to the recovered file, recovery is complete, you can now copy the file, burn it onto another DVD, or whatever.
Tip: name the BSM and SPK files using the same name as the file you're recovering. Eg. LARGEMOVIE.AVI would have files BSM and SPK files named LARGEMOVIE.BSM and LARGEMOVIE.SPK. Do this so that you do not become confused as to which BSM and SPK files are for which recovered file.
Warning: do not mix up your BSM and SPK files, this will cause incorrect data to be written to your recovered files!
Post-mortem: if you ever burn your only copy of a file onto a DVD, ensure to burn TWO DVDs. Recovery using the above process is not possible without two copies. Three copies would be even better, although recovery using three copies would be a complex process. Media Doctor does not provide tools to compare SPKs (eg to see which copy has the least errors), this would need to be done manually, if three copies of the DVD were used.
Background: you may notice patterns or patches in your BSM files, these are due to the damage on the media. A large block of consecutive bad sectors indicates that an area of the disk has failed, perhaps because the manufacturer applied an insufficient amount of coating to that area. Patterns are usually large scratches. One-off errors are small scratches and other miscellaneous damage. If you have patches of bad sectors, you have cheap disks, dump them! They will get more and more unreadable as time passes. There's no point keeping valuable data on cheap media - you may as well delete it now.
Final word: this technique is descended from an old floppy disk recovery technique, if you had two copies of the disk, when the "bad sector" error came up you simply inserted the second copy of the disk and pressed retry. This simplicity is not possible with DVDs as modern operating systems generate an "incorrect media" error if the media is switched while the "bad sector" error message is displayed. Media Doctor's ability to "read through" errors (eg. to continue without aborting when a bad sector is found), combined with its BSM and PSK functions, allow us to switch the media, just as we did with a floppy disk, but we do it at the end of the copy process, not during. This works around the error generated by switching the media mid-copy.