Center for Data Redundancy:
You might think "external hard drive" and call it there, right? Backups done?
No. Your external hard drive is probably gonna last a solid 3-5 years, depending on how heavily you use it. They're cheap and excellent and you should use them, but they're not gonna last you forever. Different media fails at different rates, so even two externals bought at the same time and likely kept under similar conditions will fail at similar rates. You have to account for the occasional damp squib too, such as a drive that fails early.
That's why I recommend not trusting any one medium, but rather using every possible medium. What is a "medium", per se? Whatever you can use for data storage, use it. Off the top of my head, here's a bunch of options...
Really quickly, I'll run down each of these, what you should look out for in terms of reliability, if I recommend them (spoiler: none of them can hurt), and their pros and cons.
(#) Hard drives
External hard drives are great. They're cheap as all fuck, they're fairly durable, they have huge capacities—these are likely to be your first line of defense in a backup solution. I wouldn't go bigger than 2TB unless you're stuffing video onto it. Not that there's anything wrong with drives bigger than that (the real theoretical max for mechanical drives is around 18TB or some such absurd number), just that you probably wanna save some money and I highly doubt you'll fill a drive unless you're a habitual video or really overboard music hoarder.
Downsides to them mostly involve their durability and limited lifespan, in my experience. If you happen to be using an external hard drive as a Time Machine drive that's always active and alive, I wouldn't count on it lasting more than half a decade. They're also fairly easy to damage, being quite literally a quickly spinning metal platter and a robotic arm in a plastic shell. Data recovery is not cheap either, though more possible than with other media.
Now, here's something to keep in mind. When you're buying storage media, you really do get what you pay for. If you go no-name for any of your storage media, you're asking for trouble. Trust big sellers on places like Amazon and trust names you recognize. The big names in mechanical drives are Western Digital, Seagate, and Toshiba. I've seen some other names around too—like my Silicon Power 3TB external, which has worked fairly alright over the ten months I've owned it—but take them as you will.
As for hard drives, it's really hard to say which are the most or least reliable, but anecdotally (by that, I mean according to BackBlaze and some other lads), Western Digital and Seagate's drives fare the worst. Still, I've owned several drives from both with no issue, so that does not mean to write off either brand. It just means that some models fail more than others, and for a storage company like BackBlaze, they need to make sure they're getting as few duds as possible.
In fact, in their Q3 2020 stats, BackBlaze cites their 6TB Seagate models as being the oldest and most reliable of the lot.
Also remember: regardless of the brand, the drive will eventually fail. If you're getting one, get two of two different brands and copy all relevant data to both. Only plug them in as much as you need to, as the motor will be the first to go on them, most likely.
(#) Flash storage
Unlike in a hard drive, flash storage uses billions of tiny little memory cells to store your data. These have the potential to be read an infinite number of times, are generally faster than a mechanical drive with a read head and a motor, and are more physically resilient. Not to say they can't fail—but flash media is usually rated for write cycles rather than age. In other words, writing to the drive over and over is how it'll eventually fail, not age. You likely already own a flash drive (pen drive, jump drive, whatever ridiculous name you call it), and your computer might already have a zippy little SSD in it.
Capacities for flash drives are only getting larger—and cheaper for the price. I remember when 4GB was the norm and 8GB was impressive—and as of writing this, Amazon's top seller is 128GB for $15. Downside is that you might lose it. I'm not kidding, I've lost every single flash drive I've ever owned. I would invest in some sorta carrying case with it, especially if you have multiple.
Tons of companies make flash drives, and there's a lot of dodgy ones out there. I generally trust SanDisk more than anyone else (they're definitely the market leaders for a reason), but I bet Samsung makes good ones too. I have one that isn't SanDisk and it's a bit crap—it works, but there's a long, irritating "pause" during reads and writes. I think I got it on campus in a blister pack, so yeah, just because it was bought in a store doesn't necessarily make it any higher quality.
Now, I think memory cards have been a rather overlooked backup format for a while now. They're actually a little cheaper than the equivalently priced flash drives, usually—downside being, you need a reader for them. Thankfully, a lot of OEM computers have them, including Macs, and USB readers are cheap if not. Go for a class 10 or better (Amazon's recommended at the moment is a class 10), and again, buy a carrying case. Here's mine, works fine.
In short, flash drives are good for quick and simple data transfer, and you can't turn one down for keeping an extra copy of your work.
(#) Optical media
Optical media—CDs, DVDs, now Blu-rays—used to rule the roost as far as distributing data goes, both personally and commercially. Nowadays, aside from DVDs well outliving the VHS in total lifespan for distributing movies, it's all gone away a bit, but fear not! I think there will certainly be a reappraisal at some point, because these shiny little discs from the future are still very very good for storing specific chunks of your data.
There are three major ways data is written on optical media. Pressed discs are the kinds you buy in stores—the factory takes one disc, creates a glass master, then a stamper which literally presses the data onto discs. These last ages. Although there's a lot of talk about how long they last, and how you handle them matters infinitely more than anything on the factory's end (don't leave your good discs in a hot car, please), I own 80s CDs that still hold up just fine. My sole optical disc to ever fail on me was a copy of The Black Keys' Magic Potion, which was from 2006.
Recordables are what's more relevant to you. These replace the standard aluminum data layer on a disc with an organic dye that your drive's write laser burns information onto. Yes, literally melts information into the disc, it's kinda metal. These age a lot worse than pressed discs, about 10-15 years anecdotally (can confirm over here), and bad handling only makes it worse. Still, one good burn stored in a dark, cool place should outlast a mechanical hard drive. (Do stay away from rewritable discs, your CD-RWs and the like—these use a weird phase-change metal that can be rewritten to, and while nifty, they last even less time. You shouldn't need to rewrite a backup disc anyway, it's storage.)
Again, please stay away from the no-name brands. You're almost certainly gonna get a bum disc if you do, perhaps even directly after burning. Verbatim is the king of the recordable media, and I've had good experiences with Memorex and Maxell recordables too. HP seems to be an alright brand too, but weirdly, my spindle has had a bunch of bum blanks right out of the box—Windows Explorer wouldn't even write onto these. The ones that it would write onto work nicely though, and these are my current somnolescent.net backup discs.
As for CDs, DVDs, and Blu-rays—go big or go home! I only use my CD-Rs if it's music or a very small dump of data, a few hundred megabytes, so their usefulness is a bit limited for bigger backups. DVD-Rs are excellent, especially dual-layer DVD-Rs which can hold 8.6GB of data over a single-layer 4.7GB DVD-R. (My HP blanks are dual-layer.) A dual-layer DVD should be more than enough for the majority of projects (like an album's worth of raw session material, a year's worth of art projects, certainly a lifetime of text and document scans), but if you have the need, Blu-ray recordables (BD-Rs) are sublime. A single-layer disc holds 25GB of data, and you can get up to 128GB out of a quadruple-layer disc.
Of course, keep in mind that if you bought a new computer that decided to treat optical media as week-old newspaper and came without a drive, you'll need one capable of burning. Blu-ray burners are the most expensive upfront, but I'd consider their capacity well worth the asking price. I also hear BD-Rs are made of better quality material than a lot of CD-Rs and DVD-Rs, so if that holds true, they might be even more worth it for keeping archival material around.
(#) Cloud storage
Of course, the most convenient option is to pay someone else to host your data. Windows constantly bugs you about using its cloud storage solution, OneDrive, though there's tons of others you've probably heard of. Dropbox has been with me since I was in middle school, while Google Drive is another solid solution, pending how you feel about Google. These are more oriented towards syncing files between machines and "always having your data", and they usually have a free tier of a few gigabytes and then a paid tier up to a terabyte or more.
What makes cloud storage such a wildcard in the arena of data storage is that the companies involved all have additional perks and client software that can do more than simply keep extra copies of your work. Automatic backups are the obvious one, but several also offer versioning of files, which allows you to return to an earlier draft of something you've been working on. Whether or not these features are worth the asking price to you depends on your use case.
Even stronger still are the cloud backup solutions meant specifically for keeping data safe—BackBlaze, IDrive, and Carbonite being big players for personal use. These are priced competitively—all-you-can-store for as little as $6 a month. Naturally, what you gain in convenience, you pay in price. A homemade backup solution, while less featureful and more taxing on you, is certainly not gonna cost the yearly billing rate of $70+ you need to pay to use. If you stop having tons of data to store, all that unlimited data might stop being worthwhile. Not to mention the need for good, fast, constantly stable internet, which might be fine if you live in a big city, but not necessarily out in the middle of Minnesota.
In general, cloud storage might make doing backups more convenient, but hot take, backups really shouldn't be convenient, because it means you're more likely to forget to keep your data redundant. Just having stuff in the cloud doesn't mean it's safe, after all. Not to mention, sending stuff over the internet naturally introduces the risk of snooping, cracked encryption, or a data breach that exposes whatever weird freaky porn you're backing up to the wider internet. Personally, for most of my stuff, it's just not for me.
My recommendation? Get the free tier of Dropbox or OneDrive and keep your really important stuff on it. Counter to what I just said, a copy in the cloud can automatically turn into two or three copies on other computers, and any copy of your work is a good copy. If you have a lot of data and you feel comfier letting a company handle it and potentially having it accessible through a bad password or a data breach, look into proper cloud backups.
(#) Analog media
You might overlook the power of the hard copy, but don't. All digital data is a shock blast or a fire away from being destroyed completely, while prints and printouts are much, much easier to both maintain and to save if partially damaged. They also age far, far better—consider that we still have photos from 200 years ago, books from 600 years ago, and scrolls dating back to antiquity! And yes, these all count as data. You'd be a fool not to take them into account if you're working in these mediums.
As such, if you have anything you've written that you're proud of, if you have any photos you've taken that you really like—get prints of them! Of course, black-and-white document printouts are as easy as any common household printer, so if you work in the written word like I do, get a new toner cartridge and store all that in a good, solid archival envelope. (Alas, I don't own a printer at the moment. Believe me, it's on the list.) With OCR being how it is, if you use a font that works well for it, there's a good chance you'll even be able to get a digital file out of your printouts if eventually necessary.
For images, things are a bit more expensive and perhaps not feasible for every last workpiece, but you can still preserve what you love out of the deal. You can do your own photo prints at home, if you have the requisite printer (and I wouldn't know the first thing about that, but here's an article that goes into details on making them at home). Look for services that work on archival prints especially; here's one to try. There's a world of opportunity out there, but ultimately, just get your stuff out in physical; it looks cool and you can be sure it'll survive into the next millennium.
Alas, for people who work in audio and video, you might be a little stuffed. Analog audio and video have always been plagued by noise issues, so full digital quality on the equivalent analog mediums aren't possible. (Well, unless you're recording digital PCM data to tape as data, but that's a bit silly in this day and age.) Still, can't hurt to get your music out on cassette! Can dub your own at home with the myriad blank tapes you'll still find cluttering up the thrift stores and perhaps online, or you can get a professional dupe house to do a run for you. (Listen, a copy is a copy, so I consider professional dubbing services to count. At least your album's not disappearing any time soon.)
For video...are there services that'll print video to film? (Apparently so, but they're rather expensive.) Okay, so this is getting a bit silly, but there is a case to be made for physical preservation of digital media. If you want stuff to stick around, you gotta think tangible and you gotta work tangible. Working on traditional means it's much easier to keep preserved. Digital is convenient, but it's definitely not the be-all end-all of creative work.
(#) In summation
Here's a table I cooked up with the cliffnotes. My recommendation: if you can get your work onto it, do that. For what I recommend the most, external hard drives for big stuff (and have two of each for redundancy), flash drives for important stuff, and optical media for regular backups. If you are in the market for backup media, don't buy it until you're sure you're gonna use it. Even unopened, blank discs still age, as does any other piece of technology.