How common is record retention on CD or DVD? I thought many backups were on hard disk and/or tape.
Data is normally kept on tape - true. But most OS or system image software is currently designed to burn ISO images to CD or DVD.
Have you ever had stuff on a CD or DVD?
The recording is made on a laser sensitive dye backed with aluminum. Take a CD or DVD and put a piece of tape on the label side. Leave it a day or two then remove the tape. The tape will pull off the label, and aluminum layer and the disk will be shot. A big gouge on the record side is easily buffer out and repaired, a tiny scratch on the label (like writing with a ball point pen) will render the disk un-usable. The atmosphere can slowly corrode the aluminum right through the label, rendering the disk useless in a few years. Any time you record a CD or DVD make two copies because you don’t want to depend on these disks to archive data.