Other World Computing's OWC Blog has revealed that Apple has severely limited the ability to upgrade the 2011 iMac's hard drive in the main drive bay. There have been various issues from 2009 on due to the way Apple utilizes an internal temperature sensors in hard drives. The problem is much worse in the 2011 iMac.
From the OWC Blog post,
With the release of the Late 2009 iMac, Apple changed the way the iMac communicates with the drive for that heat-sensing information. Each brand of hard drive Apple used had its own specific thermal sensor cable which connected to certain drives that featured internal temperature sensing. We found that you could still change the drive, albeit with a limited selection. Seagate drives could be swapped with larger capacity Seagate drives; Western Digital could be swapped with other Western Digital Drives; and so forth. There were also reports of other workarounds which included replacing the internal sensor with an external sensor (like the one from the optical drive bay), controlling the fans with software, or purchasing a replacement cable that matched your brand of new hard drive. In any event, there were perhaps a few convoluted ways to upgrade your iMac’s main drive outside of Apple’s offerings for greater speed, more capacity or to quickly restore a machine from a drive failure yourself.
In the new 2011 models, things are even worse.
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-pin power configuration to a 7-pin 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 machine’s 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).
As usual, OWC went through everything very carefully to see if there are any workarounds.
In examining the 2011 27″ iMac’s viability for our Turnkey Upgrade Service, every workaround we’ve tried thus far to allow us to upgrade the main bay factory hard drive still resulted in spinning fans and an Apple Hardware Test failure. We swapped the main drive out (in this case a Western Digital Black WD1001FALS) with the exact same model drive from our inventory which resulted in a failure. 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.
Reading the whole OWC blog post is well worth your time. This is the time to provide feedback to Apple and complain about this major limitation. Hopefully it can be solved with a software update but Apple needs to know what you think. We have been on the verge of recommending the new iMac models in place of Mac Pro systems for many tasks but this can be a major show-stopper. We were all very excited about Thunderbolt on the desktop but this is a big problem if you have any desire to replace the stock drive with a larger one such as the Hitachi 3TB hard drive which is a size not available at all from Apple.