how good is the data reliability with CD ROM and DVD RAM?

[Peter Corlett]

On Sat, Jul 21, 2018 at 05:14:00PM +0200, Carlo Pisani via cctalk wrote:
[...]
> They complained their data completely lost after 5 years of storage in CD
> ROMs, pointing out that their CDs were perfectly conserved and kept clean
> without scratches, but all the data is gone lost since the media is
> unreadable.

The lack of scratches is a red herring. CDs and DVDs are expected to get
scratched in normal use, and contain multiple levels of error-correcting codes
to protect against it. Cleaning can scratch them, so don't bother until the
discs are so filthy that the drive can't read them any more. Scratches along
the track corrupt more bits than radial scratches, which is why you should
clean them hole-to-rim rather than in a circular motion.

(Ordinary) recordable discs contain light-sensitive dyes which are affected by
UV. Some dyes are better than others, and some dyes are even so crap that
they'll self-erase in time without UV. So if you buy really shonky cheap media
and/or store it somewhere that's not lightproof, it'll become unreadable in a
matter of *months*.

> I have a lot of backup here stored in CDs, and I have recently bought an SCSI
> DVDRAM unit to create new backups in caddies DVD-RAMs (of 4.2Gbyte each)

> what is your experience?

If it's a backup, long-term durability isn't too much of a concern since under
normal circumstances you will never perform a restore, and you should be
backing-up often enough that there will be multiple copies anyway.

If you are making an archive copy for long-term storage, buy two different
brands of good-quality media, burn a copy to each, *verify them*, and then
store them in multiple locations in a lightsafe container. Ideally, re-copy the
discs every five years or so to make sure.

Counterintuitively, DVD-R is more durable than the less-dense CD-R; I'm utterly
unimpressed with my BD-R experience, finding that some disks were unreadable
after mere days.

For my backups, I just use whatever external USB drive is the cheapest at the
time. I have too much data that chopping it into 4.7GB chunks and swapping
discs is just impractical.