Cost Effective Long Term Archival of Video and Image Data – 50 TB Solutions

I wouldn't trust important backups to any single device for any significant length of time.

I've had plenty of CDs that couldn't be read after a while. (Cheap ones, admittedly, but I'm leary of the longevity claims made.)

I've had hard disks silently corrupt data.

I seem to remember I've even had SSD failures, although with a low number of writes I'd expect them to be pretty reliable.

Aside from all of these things, using a single copy means you've got no protection against physical disasters: fire etc. If you have multiple copies, you can separate them physically. Ideally I'd take some number (e.g. 3) of copies and run a checksum (I usually use MD5) periodically over everything. If one of the copies becomes corrupt in some way, if you've got multiple other copies you should be able to trust the majority, and create a new backup to replace the corrupted one. (Of course, if you keep the correct checksums in a separate place, you could trust even a single backup which still gives the right checksums, as the canonical source for replacements.)

Of course, how much trouble you go to depends on the value of the data. My personal home data is only backed up on a RAIDed NAS. My work data is in Google datacenters, which I trust fairly strongly :)

Hard drives are unsuitable for anything other than short term archival storage. The problem is not one of data retention, it's the fact that stored drives have a bad track record for not spinning up.

Contrary to what Vilx- wrote, many of us know what happens to a drive that's been stored for 20 years - they won't spin up 90+% of the time. "Modern" drives are in fact worse than the drives of a decade or two back. CDs have been around more than long enough to know that the organic dye layer of an RW disc deteriorates surprisingly fast, especially when exposed to ultraviolet light (e.g. fluorescent lighting) and temperature fluctuations.

One thing that's often overlooked when comparing hard drives to tapes is the simple fact that backup tape technology is specifically designed to store data for extended periods. Hard drives are a temporary medium only.

Edit: The following was posted as an answer to a newer version of essentially the same question, so I'll merge it in here rather than have a second post

Hard drives are designed for relatively short term data storage. Quite apart from the mechanical issues there's the fact that magnetic media, and I mean ALL magnetic media, does deteriorate with age, even under absolutely ideal conditions. This doesn't require any external influences as magnetic particles are affected by others near by and in modern drives the density is nothing short of astonishing.

A drive in use has the magnetic strength regularly refreshed, either by actual writing to disk or by the drive's own low level system, which periodically reads and rewrites sectors. A drive in storage or even one that's simply powered down, receives none of that refreshing of the data.

As for how long a drive can be stored without becoming at least partially unreadable is an ongoing topic of debate and will certainly vary even across drives from the same batch, let alone different models or manufacturers. I personally would never rely on a device specifically designed for short term data storage for any significant length of time. Even tapes, which are designed for relatively long term data storage, should be refreshed ever few years.

With all that said, I don't know what your alternative options are. Gold layer CDs/DVDs are currently being claimed to have 20+ years of safe storage but I can also remember when the exact same claim was made for burnable CDs. Those early ones proved to have a safe life of less than two years.