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I suffered slow performance for 2 years and thought the machine was just getting long in the tooth. I created a Yosemite boot disk on a USB drive. The machine was incredibly fast when running even over USB 2.0, so I had a feeling it was the hard drive. I ran SpinRite for a day and it started to show errors as well as being generally very slow. Without good diagnostics I was not 100% certain it was the hard drive, but I was almost so.
The repair was straightforward thanks to MJ's video. The only challenge I had was the hard drive thermistor, which had lost its stickiness. I used a piece of electrical tape and I hope that will work. I think that piece of plastic might be a good offering to go along with this repair, or perhaps iFixit has a tried a trusted method of sticking the thermistor? I got a bit careless when I hit this issue and forgot to plug in the thermistor, resulting in the fan blowing at full power ... so I kind of got to do the repair twice.
Take it slow. Have the cloth and cleaner on hand. As soon as the glass comes off the LCD will start to get dusty. Be careful screwing in the LCD connector's #6 torx screws. Don't over tighten. Deep breaths.