Back when we started to use CDs for backups I was horrified when I found that in general those CDs started to become unreadable after only a year in storage. We never had any problems with original CDs (say, the tons of CDs that came with an SGI computer), but the consumer-type writable ones didn't last at all. I think the technology got better after some years, but I still wouldn't trust a writable CD or DVD to last. For flash, at least, they are actually spec'ed.. and they don't last. They can't. Every bit holds a charge.. which will dissipate, eventually.
This is because commercial optical disks and recordable optical disks are actually completely different storage mediums.
Commercial optical disks, as in the disks that you got when you bought games and movies and the like, are physically stamped. The data is recorded as physical pits that are stamped onto the disk from a master copy. That disk isn't degrading unless it suffers severe external physical damage.
By contrast, recordable optical disks write their data using inks and dyes that are burned by a recording drive's laser. These inks and dyes can and will degrade over time, eventually leading to data loss.
True in general but there are, as always, exceptions. Some more expensive recordable discs use really stable dyes and remain readable for a very long time. On the other hand, some commercially pressed discs also suffer from bit rot.
I have some mid-90s CD-based games that have become partially unreadable due to that. Some CD pressing facilities are known to have produced lots of discs at certain times that are now faulty.
Just like floppies got their bad reputation from the throw-away disks and drives which hit the market in the latter days of their era the same is true for writeable CDs and DVDs. I have tons of older Kodak Gold CD-Rs which are still perfectly readable after ~30 years. I bought these back then to use as archive medium - back when 650 MB still seemed like quite a lot of data - and can only conclude I made the right choice.
Yep, circa 1995 a "normal" hard disk was 300 to 500 MB, a whole disk image would fit on a CD, we had an external CDRW drive (yes, parallel interface), soon after 1 GB to 2.1 GB become common, and I remember making multiple "data" partitions around 650 MB in size because contents of one such partition would surely fit on a backup CD.
The CD's (media) in those times were surely more durable, but also the actual drives were much more robust.
I still have somewhere a (SCSI) CDRW drive using one of those "caddies" that can read at 1x that could sometimes read old CD's that more modern drives failed at (used to, it's a lot of time I don't use it).
