FLASH media - fade … there was a recent article about SSD fade.
Hard disk - mechanical, electrical and magnetic damage, component age failure.
Magnetic media (eg. tape) - magnetic issues, aging of tape, obsolescence or misalignment of drive.
CD/DVD media - aging issues.
Putting away a “forever” copy is not a good idea, backups need to be cycled - for live data, Grandfather, Father, Son - and for retained data, a test read and reserve copy - ideally hold two copies of different age or media type at a secure location and an additional local copy.[/QUOTE]
Forever copies do not exist. Every material degrades, though more expensive and rare metals tend to degrade after a whole lot of time.
The best forever copy strategy is kinda like this:
Make 2 copies on each current latest generation media technology and 2 copies on previous generation media technology. Do this ever year. It also makes sure that you have the best chance in actually have a reading device to read your data.
As of today, i think that would require a quality ssd. a quality usb device and a quality sd card for current media technology. For previous generation it would require writable discs, tape and an old usb drive.