From the tutorial we got the cooling holes on the front and right side.
I'm pretty sure the hard drive also has holes on the left side, but those do not get mirrored as the boolean modifier comes after the mirror.
Of course I can just copy the hole punch cylinders and manually copy them to the other side. I was just curious if there maybe is another way to "just" mirror them on the X-Axis
As it is an X+Y Mirror this does not work, but after reading your answer and thinking about it I got it:
Adding a X-Axis mirror to the Cylinders that are used for punching the holes in the sides and setting the Object in that modifier to the body.
