HDD behind motherboard?

I'm planning a build around the Fractal Define R4 which has two 2.5 mounting bays behind the motherboard. My plan is to buy a 128GB SSD drive and  a regular 1TB 2.5 drive as well and mount them behind the motherboard panel. What I'm not sure of is will the regular HDD will get too hot. Any idea's on whether this will work or not?

Do not mount the HDD behind the motherboard

Ok but why? Are my concerns about the heat correct or is there another reason?