Easiest method: clone your drive
We're not asking you to splice some DNA and clone a living thing. Instead, do an exact replica of whatever's in your HDD to preserve everything. This is really the most straightforward method because it's just a matter of installing a cloning software, select the drive and have it mirror everything onto either another internal or external HDD.
How simple is it? If we say in a mere few clicks, you might be thinking we're yanking your chains. Trust us, it's that easy. For example, Apple's Time Machine, which is pre-installed with its OS X operating system, lets you do a direct clone of your system. You can even restore files at specific points in time.
Windows users also have a similar backup option, accessible when you click on Start > Update & security > Backup > Add a drive.
Then, there are backup software that does exactly everything you expect - saving your photos, music, movies and everything, right down to the system files, as an image file.
One such software is Acronis True Image 2016. It doesn't just create an image file of your system, Acronis True Image 2016 also has the flexibility of restoring your system at a specific point in time. Also, if you're sure you'll need only specific files, you can restore just those and leave the rest alone.
That said, we'd advise that you use this in tandem with a cloud backup service. That way, you get to have a second layer of backup, just in case the odds aren't in your favour and things go to hell while you're creating a mirror image of your system.