How do I copy all data in HDD quickly?

Earlier this week I found a folder I'd completely forgotten about that I made ~6 months ago and stored writing that I don't want to lose in.
Where was this folder?

1. Backing up files.
That's a very silly and inefficient way of backing up your files.
2. No, the size of my internal HDD is ~750 GB. The sum of the size of my files is 500 GB. Actually, come to think of it, It may be 500 GB left. I forgot if I subtracted or not, so I gotta check; it's that screen that says 'Local Disk C: ___ GB free of ___ GB'
What are these files? Games, documents, movies, pictures?
3. Then it's because of their protection, and not because of a limit to how much I can copy and paste?
Yes, some files are protected from the user.
4. Afraid I might need it in the future so I might as well store it since I have the space.
Need what? Your system files? It doesn't work like that you can't copy and paste an operating system. You'd need to use a fresh install or make a clone of the drive.