Writing your pin on your ATM card.
Keeping a copy of your front door key on your door.
And not backing up your stuff.
Keeping a single copy of your pictures, videos, or documents in one location is just not a good idea.
If that device gets lost, stolen, or breaks, then what?
Now that you're sufficiently panicked, let's talk about backup methods.
Get an external hard drive or flash drive.
So here's an external, here's a flash drive.
This is one of the simplest methods.
Buy one of these, and connect it to your computer.
Then, copy your files from your computer to these things.
Do this routinely and you'll have a second copy of your stuff should your main PC take a dirt nap.
Backup to the Cloud.
You can also backup your stuff to servers somewhere away from you.
You can sign up for a service like Dropbox, Box, Microsoft One Drive or Google Drive and have your data automatically uploaded to their computers.
They all offer a free storage tier, and you can purchase more space.
And number three, use both.
A belt and suspenders approach won't hurt either.
If you're looking for recommendations for external hard drives, or cloud backup services, check out CNET.