[ale] ddrescue on cdrom

[jdp at algoloma.com (JD)]

On 02/24/2012 05:58 PM, James Sumners wrote:
> I don't understand this. The CD already includes Reed-Solomon codes for error
> correction. Sucking up more space on the disc with a second helping of
> Reed-Solomon codes shouldn't do anything other than reduce the amount of usable
> disc space. Especially since PARs are usually ~20% of the original data. 
> 
> On Thursday, February 23, 2012, JD wrote:
> 
>     Try a different drive.
> 
>     In the future, you might want to include par2 files with the data. This isn't
>     foolproof, but it has helped me to recover 2 failing DVDs that were not
>     completely gone.

I use 10% par2s, not 20%.  All that I know is that when files couldn't be
properly read from multiple DVD drives, I used dd_rescue to get what I could
which was also incomplete.  Then used 'par2 r' and was able to get everything
back, verified.

I have hundreds of DVDs with par2 data as added protection - some are over 10
years old and are still working.  2 have seen some failures, so I pulled the
data off, recovered it and wrote it back to new media. Good for 5-10 more years
the way I see it.

I'll leave the how/why this works to smarter people.