Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Backing up your system (and documents) is more than just copying a lot of files to another location.
Here's an example for you. Let's say you want to backup your operating system, Windows, for example. So, you copy every single file on the drive C: to an external device and then copy the contents to a hard disk on another machine. And then you try to boot the other machine.
Nothing will happen. The other system will not be bootable, even though you have copied just about any file you can see and access.
This is because operating systems do not work the way we perceive them. We see files, but these files are only meaningful to a file system that can read and interpret them. For example, the NTFS file structure is only useful to systems that can read this file system, like Windows, Mac and some Linux distributions. An old DOS machine cannot read a hard disk formatted with NTFS, for instance.
However, beneath the file system, resides the most basic structure of any device - disk sectors. These are actual, physical parcels containing the data, regardless of the file system used. If this structure is mirrored to another device, it is possible to create an exact clone. Any operating system that exists on the device will not know any difference.