How to replace the hard drive

It is pretty similar to the 20" guide, but there are a few gotchas. I just went through the process today, and I had to improvise/figure things out a bit. Your mileage may very.

• Step 3. Skipped. I tried this and just could not get anything to click or happen. Finally I tried step 4 and was able to open it up perfectly. Upon further examination, on one side there was a broken metallic mesh. Oops. Nothing important though.
• Step 8, 9, 11. There was no EMI shield/tape in my iMac. Perhaps it was removed at some point (would be interesting to find out) when my DVD drive was replaced (by Apple Care, under warranty). It looks as i was cut on purpose. This will be fairly obvious.
• Step 12. Possibly due to the bigger screen size (24"), there were actually 6 screws attaching the display to the rear case. MORE IMPORTANTLY: As noted without much emphasis in the fixit guide, these screws really are deep in. The torx screwdriver kit I got had a stub at the top to switch tops which was too fat to fit in the gap between display edge and case. Furthermore, when I got a skinnier screwdriver kit it was not magnetized.
• Do yourself a favor and try finding skinny, magnetized Torx T10 screwdriver, which is the one used in this step.
• Step 14 & 15. Skipped. No where in sight, so they really didn't get in the way. Can't really explain this one, maybe they're in a different spot.
• Step 16. Huh?? Skipped as well.
• Step 18–22. This is where everything was completely different. Everything seems to be positioned differently, I should have taken pictures.

1. The HD is towards the top-middle of the case. One side of the HD is impossible to unscrew as there is no room for a screwdriver. So unscrew the side which is accessible (also tricky if your screwdriver is too big). These "shouldered" Torx screws are fastened to a black clip.
2. After removing the two screws, you need to squeeze the clip really hard and somewhat upwards to yank it out. It is a bit tricky, so obviously don't brute force it. Once the clip is off you can slide the HD right out. This is counterintuitive as one think the other side is screwed, but the ones on the other side are smoothed, shoulderless, Torx pins!
3. Now unscrew the Torx pins and screw them onto the new HD on the side which goes into the same, obstructed place. Assuming the SATA connection of the HD is due north, the pins should be on the east side of the hard drive.
4. IMPORTANT: Step 23 sounds nice and easy, but I only really understood it in practice. Once you detach the sensor, you are supposed to "transfer" it to the other HD. Well the keyword in this step is "adhesive". This adhesive is not very reusable. Thanks for the heads up, guys. In any case, I luckily had some adhesive putty, and I have no idea how heat resistant it is or not, but I used that. (The original HD was also a lot larger than the new 3.5" HD, so the heat sensor was actually a bit larger than the edge of the HD. No biggie though, I just had to center it properly.)
5. After "transferring" the heat sensor, slide the HD back into place. However, you need to hold the HD plus the black clip while you put them in place, and at the same time screw in the shouldered Torx screws. Someone helped me hold the HD, mind you.

And that's that. Just backtrace and all will go well. It's actually easy to put back together than to take apart. Good luck!