Yes, they will overheat really easily if you keep then in the enclosure purchased with the drive.
I initially suggested external ones as they're slightly cheaper and the casing merely pops away from the drive. Then, put them into a NAS enclosure or an old server and that will keep them cool enough to function.
Thanks for the advice mate. Perhaps, mining with HDD would be more accessible than with ASICs. HDDs can be found even much cheaper than ASIC hardware, in my opinion, and more accessible and easier to find. For cooling, I had this crazy idea of using USB powered fans connected to a USB Hub to cool off the HDDs.
The USB powered fans would work, but they'd have lots of wasted energy involved (and you'd have to position them so they allow air to flow in one direction through the entire drive).
HDDs are definitely cheaper, easy to source and quite cheap to fix (but asics can potentially produce more profits).
Is this mining with the Raspberry Pi or some stick miners?
This is more of mining with external ASICs (or stick miners) with the Pi. Also, it is possible to mine coins with the free space of your hard drive. The coin is called BURST (among many others like Siacoin) which allow you to mine them with your storage device. I will post pics of my Pi with the other equipment doing mining tasks once I finish ordering all the required hardware (and finish this college semester).
Also @abiky, I'd suggest you test the function on a regular computer to check you can set it up correctly.