A lot is written about the importance of backing up data, but the media and methodologies proposed aren’t generally suitable for archiving. Securing your data for posterity, i.e., archiving, requires a different approach, where shelved media life and future file compatibility trump the speed and convenience that make backup palatable to the average user.
External hard drives
Portable hard drives are easy to use, faster than optical, but may need their data refreshed every few years.
By far the most common backup media employed is the external hard drive. Its fast compared to tape and optical, hard drives are generally reliable for the short term and can have a large capacity i.e. 8Tbytes or more, and if removed from operation and safely stored, may last a decade or two before magnetic properties diminish to the point of producing unrecoverable errors. For the long term, hard drives on the shelf are workable, but require periodic maintenance—so they are not ideal.
For non-operational drives, it’s industry practice to refresh, i.e., rewrite the data every two or three years. This is called scrubbing. Each file is read then written out again to the same or a different hard drive.
It is also good practice to keep more than one archive disks, use them in pairs or trios—each containing a copy of the same data or on another media so failed reads can be recovered.
Environment is also key: Heat, vibration, humidity, and magnetic fields (strong ones are used to erase hard drives) can dramatically shorten operational or shelf life. A hard drive is also a mechanical device that’s vulnerable to shocks.
External SSDs
Portable SSDs fast, faster than mechanical hard drives but have less capacity than mechanical hard drives for the same cost. however, most NAND-based storage is only good for about a decade or so.
External SSDs are rugged and virtually shock-proof, but the NAND they use won’t hold data forever. The cells, which are electron traps, leak over time. The technology is also relatively new, so no one is quite sure how long an SSD will retain data when stored unpowered, but you won’t find companies touting them for long-term backup. Figure 10 years as a best case scenario, but don’t rely on it.
If you use SSDs, refresh or scrub the data on them every year or two and replace them every 10 years in the same as mechanical hard drives.
Tape
Magnetic tape is still one of the best backup media for enterprise. It’s available in very large capacities for example, a new Sony type can hold up to 185TB. It’s also removable media, so it’s easy to store and handle in bulk. But tape can stretch and break, as well as be erased by magnetic fields. It’s also expensive; the handling mechanisms are finicky; and because data is stored sequentially, random retrieval is quite slow. It also suffers magnetic and physical degradation over time, though the rate is greatly dependent upon the materials in use. So, don’t use tape. It’s expensive and there are easier alternatives.
Optical
The media must be inerasable: According to the law of data entropy, whatever can be erased, eventually will be erased. Media and mechanicals must be separate: Hard drives hold a vast amount of data up to 18TB on a single drive but like other mechanical devices with moving parts, they can break. A well-made, properly stored CD or DVD frees you from having to worry about mechanical-component failure. If you expect moving parts to last for decades, you'll be disappointed. The media must be inexpensive: The cheaper copies are, the more likely you are to make multiple copies, which in turn will increase the odds that at least some will survive. The media must be ubiquitous: If everyone uses the medium now, chances are better that someone will be able to use it in the 22nd century or at least later in this century. The media must be robust: It needs to survive for decades. CD-Rs, DVD-Rs, and DVD+Rs meet the first four criteria, but despite various claims of longevity based on laboratory aging no one knows how robust they'll be in the long run. Representatives of leading data-recovery companies DriveSavers and Ontrack told me that they occasionally see optical discs with symptoms possibly related to age and poor manufacture, but that it isn't a common problem. Your best bets among optical discs are relatively expensive archival discs such as Delkin's Archival Gold, Kodak's Gold Preservation, and MAM-A's Archive Gold. The makers of these discs claim to use higher standards for them than for run-of-the-mill data discs; and all use gold rather than silver in the disc's manufacture to increase longevity . But archiving concerns extend to other issues such as whether the file format will be readable in 50 or 100 years. Your chances are better if you stick to popular formats such as .jpg, .mp3, .doc (but not .docx), .txt, .html, and .pdf. And the more formats you can save the file in, the better. You should store the discs in jewel cases, upright, away from direct sunlight, humidity, and extreme temperatures.
Blu-ray recordable discs are rated for 50 years. If you think of optical (CD/DVD/Blu-ray) solely as a means of movie or software delivery, it probably seems antiquated. You might also dismiss garden-variety CD, DVD, and BD-R (LTH) recordable and rewritable discs as unsuitable archival media. You’d be right about that—they use inherently unstable, organic dye-based data layers.
However, there are optical discs that are unquestionably the hardiest, handiest archival media available to consumers. Write-once BD-R HTL (High To Low) can last for 100 to 150 years given a relatively mild environment—i.e., not on your dashboard in Phoenix. Milleniatta’s M-Disc BD-R and DVD+R write-once discs use an even more stable data layer that is rated for 10,000 years. Only its polycarbonate outer layers reduce that to a mere 1,000 years. Note that this is all theoretical, but the testing MOs were rigorous and performed by the government of France (BD-R), and the Navy for the Department of Defense (M-Disc DVD).
Available in 25GB, 50GB, and 100GB (currently very expensive) flavors, BD-R also has enough capacity to handle long-term backup and archival chores. The downside is a relatively slow 21MBps writing at best—substantially slower than USB 3.0 hard drives and SSDs.
If you’re worried about optical drives disappearing, know that optical retains a very strong presence in the archival community, as well as the enterprise, so that should give you some reassurance.
Despite its slow speed, optical is pretty perfect for archiving your most important data.
Online storage
If I were big on blind faith, I’d just say opt for online storage and be done with it. It’s super easy, convenient, and there are some very cheap online storage services such as Amazon’s Glacier, BackBlaze, Google Drive, and OneDrive. Glacier is extremely inexpensive, at least until you need to retrieve data.
However, there are drawbacks. First off, though the means of delivery may seem magical and your data is often referred to as being safely stored “in the cloud,” in reality, it’s stored on someone else’s hard drives or other media. It’s as safe as a given service has made it.
Then there’s the ongoing cost in the form of monthly fees, and in some cases transfer charges. Also, speed and availability are limited by your online connection (DSL often has very slow upload speeds) and when your service is down, your archive is unavailable. There are also privacy and security concerns. I consider these trivial, but just FYI—the NSA had a hand in funding just about every open-source encryption project out there.
Caveats aside, having an offsite copy of your data is one of the mantras recited by backup and archival gurus. If a flood, hurricane, fire, or ex-spouse ruins your local backup, you’ve got another to fall back on.
If you use online storage, use it as a partner to local backup. That said, it’s a lot better than not archiving at all.
Conclusion
Pick the one of the above and with a little luck, your great-grandchildren may enjoy your mementos from the early days of digital photography.