Please Note: This range has a common problem caused by removing or changing the internal hard drive whereby the internal fans run at maximum speed all the time.
It is typically referred to as the 'Runaway Fan' issue. On the 27-inch ranges listed below, the internal hard drive's SMART interface communicates directly with the internal logic board over the SATA connection. Unfortunately, Apple has proprietary firmware installed on the drives it uses on these ranges meaning that fitting anything but an original Apple hard drive for/from one of these ranges will prevent the iMac from being able to get a meaningful temperature reading and will trigger the fail-safe of running the fans at full speed.
The two 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 SATA hard drive/SSD
The first solution becomes a problem when you want to go beyond the drive capacities or speeds that Apple originally provided, such as 4-6TB drives, or 2.5" 6G SSDs (which can be used along with an adaptadrive). For a list of our currently available new and used original Apple iMac drives, click here.
The OWC in-line sensor cable is a more versatile solution and can be purchased here. Once fitted, you can use almost any SATA hard drive or SSD in your 27" late range iMac (2.5" devices will need an adpter like the adaptadrive).
The late 2009-2011 iMacs suffered from the same syptoms but the solutions are different for those ranges. See this link for more details on the fault, as discussed relating the 2011 models: Blog Link to HD details