No matter what method you choose to store your files, they can (and probably eventually WILL) fail. Hard drives can fail, cloud storage companies can (and have in the past) disappear overnight, CDs and DVDs can degrade and become unreadable over time (archival quality discs minimise that risk but cost more than regular discs). RAID systems seem safe but multiple copies across the system don't help if the controller goes and you lose everything. And of course, even if you did have some amazing form of storage that was never going to simply fail, there are also the risks of disaster such as fire, flood etc., destroying your house or just a simple burglary where somebody steals your computer and the backup hardware.
So, the rule is multiple backups in multiple locations and check them from time to time for integrity.
Some people also say you should use more than one method, perhaps having a cloud storage setup as an off-site backup, that is a matter of personal preference. My personal choice is similar to Bassats, multiple external HDDs. After a shoot the cards are downloaded onto my main drive and my main on-site backup drive, as well as a portable drive, before the cards are formatted ready for my next shoot. That gives me three sets of the files from the shoot, one plugged into the computer, one that gets unplugged and tucked away in another room (in case of theft, a burglar is unlikely to find the hidden drive) the portable drive goes in my car and when I visit any of the 3 friends who keep an off-site backup drive of mine, any recent stuff is copied to their drive. As I edit the work it also goes through the same backup process, admittedly there might be a couple of evenings work sometimes sat only on the main drive until I get around to plugging the other drives in to backup, but worse case scenario is I have to redo a few hours work after restoring the original files from a backup. Nothing irreplaceable is lost, I always have the originals to fall back on. More often than not though, I plug one of the backups in while I work and backup edits as I do them
Once I have finished working on the shoot all originals and edits (and often stages of edits) will be stored on 6 drives (2 at home, 1 in the car and 3 scattered around at friends homes). In the event of my main drive failing it takes me a couple of minutes to dig out the backup drive, plug it in, and carry on.
As Bassat said, drives are cheap, sure one may fail from time to time (or, in my case, get knocked on the floor whilst running) but it takes almost no time to switch to a backup and carry on. Then, buy a new drive (or send it back for a free replacement under warranty) and transfer all your files over from the other drive.