Unfortunately, taking the computer apart -- which is challenging enough -- is not the only obstacle to upgrading the hard drive in these Macs.
OWC also discovered that the "Late 2009" -- and subsequently introduced "Mid-2010" -- models use a "connector that seems to use the drive's internal sensors" rather than an external sensor like earlier 20-Inch and 24-Inch Aluminum iMac models.
This means that the most straightforward way to upgrade the hard drive is to "replace the drive with another model from the same manufacturer that [OWC or another third-party has] confirmed works properly with this thermal sensor cable". OWC provides a list of compatible drives. Readers have shared reports that taping an external temperature sensor to a hard drive or SSD that does not have an internal sensor will work in these models, but this method could be risky when data is important.
As again found by OWC, if an SSD is not installed at the time of purchase in the 27-Inch "Mid-2010" models -- the iMac "Core i3" 3.2 27" and "Core i5" 2.8 27" -- the connector cables and mounting bracket are not present. It still is possible to install a 2.5" SSD in addition to the hard drive if an SSD is not installed initially, but one will have to find a way to mount it place. It also is worth noting that if a 27-Inch "Mid-2010" iMac is configured with an SSD, but without a hard drive, the hard drive temperature sensor is not present either.
For the "Mid-2011" models, OWC once more touched off a firestorm across the blogosphere by reporting that Apple has gone even further in the company's effort to restrict hard drive upgrade options:
For the main 3.5" SATA hard drive bay in the new 2011 machines, Apple has altered the SATA power connector itself from a standard 4-wire power configuration to a 7-wire configuration. Hard drive temperature control is regulated by a combination of this cable and Apple proprietary firmware on the hard drive itself. From our testing, we've found that removing this drive from the system, or even from that bay itself, causes the machines hard drive fans to spin at maximum speed and replacing the drive with any non-Apple original drive will result in the iMac failing the Apple Hardware Test (AHT). . .
We've installed our Mercury Pro 6G SSD in that bay, it too results in ludicrous speed engaged fans and an AHT failure. In short, the Apple-branded main hard drive cannot be moved, removed or replaced.
In regard to 2009 - current iMac's and their thermal sensors on their hard drives...
Apparently (after talking with OWC tech support) any standard 3.5" hard drive will work as long as you use some sort of fan control software which reads the hard drives temperature through S.M.A.R.T. which is passed through the SATA data cable and not through the two additional leads that Apple uses with it's original drives.
This is the fan control that OWC recommends (free).
Yes, this is a SATA hard drive that is the same size as the one currently in your iMac which makes it compatible. The only issue that you might run into is the hard drive bus in your computer is a 3 GB/s connection and this hard drive has a 6 GB/s connection. I have installed 6 GB/s hard drives in 3 GB/s computers and it works fine you just don't get a 6 GB/s speed when reading and writing data. Other than this the hard drive should work fine.