Thank you for your help, Zerubbabel and crashdaddy!
I just found out that I can upgrade my internal hard disks and use a second SSD instead of my HDD, and I can also upgrade my RAM. But I don't know whether it is still worth the costs, because the laptop is already almost five years old.
So using an external SSD hard disk or moving folders from C to D might be the better (and less expensive) solution for my first game.
If you want to replace the existing HDD, a 500 GB SATA SSD could be swapped in for about 20-25 bucks so it's not really a big investment. You could also move said drive to a desktop later.
Also, if you're looking to clone an existing drive, say if it has your operating system on it and can't easily reinstall it, an application called "Macrium Reflect" works very well. It's technically a paid software, but there is a free trial that turns into a limited free edition once the trial is over. I recently replaced my dying ssd with my OS on it, and it was painless.
