Apple introduced in the 2009 iMac a new system to monitor the temperature of the Hard drive without going through the SMART status, by directly getting the information from the sensor inside the disk.
Each hard drive manufacturer had a different plug, but it was quite easy to shortcut the two wires to be able to install a disk of a different brand.
OWC discovered that APple modified that system in the 2011 iMac. It is now integrated to the hard drive power supply cable.
That's not all. It was also discovered that each type of hard drive has a specific firmware allowing the disk to use the sensor and that the models available at third party retailers have problems with the sensor (we will get to that problem later). The worse part is that there is currently no solution to deactivate the sensor.
All this has several consequences. the firs one is not very serious, it is the fact that the hard drive doesn't pass the hardware test successfully because of an error on the disk. The second is more serious as the fan located near the hard drive is constantly running at full speed if the hard drive is replaced.
Therefore, it is currently impossible to replace the hard drive of the 2011 iMac without going through an Apple Retailer to get an identical disk, which of course will be sold at a price three times higher than anywhere else.
OWC is heavily criticizing Apple for this new evolution and even accuses Apple of implementing a planned obsolescence policy. It is true that with this new evolution, OWC is losing part of its business, the replacement of Mac components.
For once, we will not be as harsh against Apple. the main goal of this technology is to monitor the temperature of the disk as precisely as possible. It is absolutely necessary in a confined architecture such as the one of the iMac, and it is the only way to keep the fans running at the right speed to cool down the disk while keeping the computer quiet, without the fan blowing at full speed all the time.
We are absolutely sure that there is a way to deactivate or to trick that sensor, all that needs to be done now is to find a solution, and that is of course something we will try to do.
[Update] It didn't take long to find a solution:
It is the solution provided by Apple on the models coming with an SSD: short-circuit pins 2 and 7 on the power supply cable.
Here is also an explanation of the reasons that pushed Apple to introduce that system in the 2009 iMac. As we sais earlier, the iMac checks very often the temperature of the hard drive. If it was doing so with the SMART system, it would shrink the disk bandwidth and would freeze the disk for a very short time at every check.
Therefore Apple decided to do it with an Out of Bandwidth system, outside of the data channels of the disks. At first the company used connectors added to most hard drives and used for programming and testing the disk. That solution had one drawback as each manufacturer had its own connector. The 2011 model uses a new system. All the information now goes through pin 11 of the power supply connector, which is normally used to light-up a LED during disk activity. In prodder to change the use of that pin, a specific firmware was needed, which explains the reason that only disks sold by Apple in the iMac don't have any problem.
It was quite a radical choice; far from established standards, but in the end, it will not cause any big problem in the replacement of hard drives in these computers.