Recently, I met a bad situation, about iMac s.m.a.r.t error. Let me have no choice just only can do hard disk replacement.
After some research, found some great links
How:
https://www.ifixit.com/Guide/iMac+Intel+27-Inch+EMC+2309+and+2374+Hard+Drive+Replacement/1634
Fan issue:(free :)
http://www.crystalidea.com/macs-fan-control
Although above link are great, but still sometime become difficult for complete newbie, like me.
That's why I start to share my experience, hope this help you through.
1, First, check guide "How" link above to do replacement, and take care of any step.
2, After you finished to the step to put your new hard disk back, and want to return all unplugged cords back, you will found a issue. There are totally three cords unplugged from hard disk, and you will find you can only plug two cords back, for the third one, for its 8 pins for apple's customized spec, but your new hard disk are only 4 pins, OH......, you can scream a bit... it's OK ;)
So, how to fix this? and what it affects? for me, I just skip this, do nothing, let the 8 pin cord to stick somewhere is fine, no need to plug it back. This will affect the temperature sensor detective. After you skip this, your fan will run in full speed after 10 mins your machine start. But don't worry about it, second link listed above will help you through this, and it's free. So, skip to plug the 8 pins cord back and continue other steps.
It will be a better way if you can boot from a USB drive.
help links:
Create a bootable installer for OS X Mavericks or Yosemite
Starting from an external USB storage device (Intel-based Macs)
3, So, how to control the fan speed? simply install a free software "Macs fan control" then the issue is gone.
So, everything is .... OH... another scream...
4, Before you can install the software, you sure need to install OS for you new hard disk, but for me, I found nothing showed after I put OS DVD in when let me to choose which disk to install the new OS. The reason is the new disk is still haven't been formatted, but don't worry, just need a little more step, after you put the install DVD, you can see the Utilities in the menu bar, you can format disk from there, then everything goes on.
hope my experience help you a bit.
