What is the best backup arcival media, DVDs, USB or Hard Drives ?

There are many comparatively cheap technologies available for backup these days, varying from hard drives and DVDs to solid-state drives, USB sticks, NAS (network-attached storage), and even cloud storage online. However, choices become harder if you consider long-term storage that is likely to survive 30+ years from now.

When considering long-term data storage solutions, you should take into account many factors, such as:
- Media life - what is the likelihood of catastrophic failure of your backup device of choice, and possibility of data recovery ?
- Cost (per gigabyte) - different media has considerable price differece
- Security - do you trust the physical location of the media, or the company offering cloud storage with all your data ?
- Ease of use - how easily can you backup everything you need to, can the process be automated so that it does not consume your time and is not contingent on your diligence ?
- Future-proofing - is it likely that your technology of choice will be backward-compatible many years from now ?

With all this in mind, below is a list of the common devices in use today, with pros and cons to each technology.

Hard drives - moderately priced, suitable for short-term storage. The problem with long-term archival is not data retention, rather the fact that stored drives have a bad track record of not spinning up after a few years in storage. "Modern" drives are in fact worse than the drives of a decade or two back.

SSDs (solid-state drives) - expensive to use as archival media, unproven track record as long-term storage.

Optical media (DVDs/Blu-rays) - recordable/RW optical media has a lifespan of 10-20 years if stored in a dry/dark place without excessive temperature changes. Their organic dye layer can deteriorate very fast if exposed to UV(fluorescent) light and temperature fluctuations.

USB drives - they have no moving parts, they're very handy and should last a very long time, however, considering the cheap manufacturing they can be very failure-prone as well. They are generally not considered an archival media.

Cloud storage - somewhat reliable, as long as you can trust the company. It can be expensive for storing large amounts of data.

Whatever your device of choice, keep in mind that they all have limited lifespans and are prone to failure, and your best bet is making separate copies using different techologies, at different physical locations!