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.
