I have had my iMac for almost 4 years and the internal WD hard drive went kaputt. The system was hanging often, sometimes for as long as an hour. I knew something was amiss. I had to run Apple Hardware Test on an extended test in order to come up with the hard drive failure error. I have an external clone backup which is updated daily, so, I was able to boot from that drive and use my computer as normal while I waited for the replacement drive and tools to arrive in the mail.
Went swimmingly! I cleared off the coffee table in my living room and used the iFixIt app on my iPhone to guide me through the process, step-by-step. The entire process took maybe 1.5 hours, since I was being extra-careful. I used the dust blower to remove a lot of built-up dust bunnies inside the computer, and to blow any errant particles off the exposed screen.
Stay grounded. No shuffling about on carpet in your socks while you perform iMac surgery. It was a pleasantly warm day where I live, so I ran around barefoot, and without a shirt on. I felt more like an auto mechanic than a computer geek. Oh, and if you still have the box your iMac originally came in, sticking the screen glass into the styrofoam molds in there is handy, as is putting the detached screen into the padded computer sleeve. Keep a lint-free cloth around, just in case you put a finger print on the screen. Try really hard not to, though.
Universal Drive Adapter
Heavy-Duty Suction Cups (Pair)
Mako Driver Kit - 64 Precision Bits