Since my internal HD is dead I run my iMac from and external drive (I had a clone of my internal drive on this external one). But I lost speed and I decide to replace the internal drive.
With your instructions and tools it's was not only easy but fun to do that. And best of all my old (2008) iMac is now faster than ever.
THANKS A LOT IFIXIT!
Look at the video a few time and read the instruction before doing anything. When you fix your computer take care about the wires you unplug to be sure you'll re-plug those wires correctly. And with your computer «open» it's time to clean it with a gentle vacuum brush.