In article <0001HW.C53EEF77013A5B64F0182648@news.astraweb.com>,
Nelson wrote:

> I have some old (well not that old) CD-R's which have developed a
> problem which looks like the dye is deteriorating. Because it is only
> in spots, some of the data is readable. Furthermore, Toast is able to
> create a disk image of the disc on some drives. Since read errors
> don't appear to be reported unless repeated attempts fail, I can't tell
> how bad the problem is. Furthermore, I'm not sure the "read" data is
> the "correct" data.
>
> The disc utilities I have only check the file structure and do not
> verify the individual files. I presume making an image forces reading
> of all the data on the disc, but just because it can be read doesn't
> mean it is not corrupt does it?
>
> - Does the CD contain error detection data, eg a checksum for every
> sector or block?


CD-Rs employ multiple simultaneous forms of error correction. It would
be unheard of for a physical error in the media to manifest as a
readable but corrupt file.

> - Does anyone know of an application which will examine a damaged disc
> and associate damaged sectors/blocks with files?


Shouldn't be necessary given the above.

> This is a pretty depressing problem since the reason for writing the
> data to CD-R's in the first place was the unreliability of magnetic
> media.


Who told you magnetic media were less reliable than recordable CDs? The
issue of writable optical media deterioration had, I thought, been well
hashed out several years ago. The short form is that 5 years is about
the *upper* limit of reliable lifespan depending on media quality and
user handling. Some may last longer and others fail much sooner. They're
fine short-term storage but they're not archival. Magnetic tape remains
the serious solution for long-term writable storage. Regardless of your
solution, you should periodically test and recreate your media unless
you're using professional quality materials and techniques.

--
"Harry?" Ron's voice was a mere whisper. "Do you smell something ... burning?"
- Harry Potter and the Odor of the Phoenix