This range has a common problem which is caused by removing or changing the internal hard drive.
It is typically referred to as the 'Runaway Fan' issue, but it involves the internal hard drives SMART interface communicating directly to the internal logic board. But Apple being Apple, the firmware is unique to drives shipped and fitted in the later ranges.
See this link for more details on the fault, as discussed relating the earlier 2011 model:
Blog Link to HD details
(the fault is the same, but the solution is different for the 2009, 2010 and 2011 models)
The 2 solutions to this problem are:
• Fit an Apple branded original drive with the correct firmware installed
• Fit an OWC In Line sensor cable, then fit any hard drive you want!
The first solution becomes a problem when you want to go beyond drive capacities or speeds than Apple originally provided, such as 4-6TB drives, or 2.5" 6G SSDs (which can be used along with an adaptadrive
This is not a marketing or sales pitch, simply an info page to present the options for you, as we encounter this problem on a daily basis.
The OWC in line cable can be found
Once The In Line cable is fitted, you can fit any 3.5" SATA drive into your 27" late range iMac. But if you are stuck for options, please see the choices below:
• Recycled Drives
• New Drives