[GRLUG] Mounting DVDs

[Michael Mol]

On 1/21/2010 11:27 AM, Bob Kline wrote:
> Re optical media decays, yes, this is
> always a possibility.  Bad batch of blanks?
> There's a vast lore about what causes
> CDs and DVDs to deteriorate.  Whether
> rewriteable media are better or worse
> than write once, etc.

In general, since concentrated light is used to fade (burn) targeted 
points in optical media, diffuse and ambient light will, over time, do 
the same to general regions.  After a while, the difference between a 0 
and a 1 will become very difficult for a drive to discern.

That does raise the point that a high-quality drive (SONY and Plextor 
used to be the best manufacturers. I don't know who's at the top of the 
quality game, now.) could be able better differentiate between a 0 and a 
1, and ultimately be able to recover enough of the data that ISO9660's 
built-in error correction could kick in to recover the rest.

>
> But there is still the issue of the original
> machine being able to read them.  Perhaps
> that has to do with alignment issues between
> DVD units.  One hates to think about this,
> but apparently it is common enough.
> It somewhat says one should keep around
> the very DVD unit they used to make the
> backups.

See my remark about drive quality above.

>
> But better is to try them right after making
> them, one another unit if possible, which I
> suspect few of us usually do.....

Not typically, no; The verify step is normally done in the same drive 
that burned it. That's an automated step built in to every CD burning 
tool I've used in the last ten years.

>
> Thanks for the tip about ddrescue.  I'll
> take a look at it.  The media in question
> have no scratches, but there are other
> possibilities.  In the end I'll probably
> just put the DVDs in to the old unit, read
> them, move them to the new unit, and
> consider it yet one more lesson learned.

I use ddrescue to back up data DVDs my computer will mount, dvdbackup to 
back up video DVDs, abcde to back up audio CDs, and I'm now using cdrdao 
to back up things I can't mount or identify from within Linux.

>
> Your point about several kinds of backup
> is well taken.  It's all a matter of how much
> what you are backing up is worth to you.
> And of course one always has to ask
> whether their backup devices are working
> as expected...

Well taken, but not well-followed on my end. I don't even *have* a good 
off-computer backup system in place. (That was part of the reason I 
pounced on David's Xeon box; It's going to be an in-home rsync target 
for my main machine. Now to pick up a cheap SATA controller and three 
more 1.5G drives for a RAID5.)