Apple has thrown a curve ball at iMac owners with the latest 2011 “Thunderbolt” models. The 2011 models use pins on the SATA power connector to transmit SMART temperature information from the drive to the iMac motherboard (previously, the temperature data was transmitted through a separate cable). This information is only provided by hard drives that have firmware built in to format the data the way Apple expects. If you install an otherwise-identical hard drive, the fans of the iMac will rev up to maximum speed and the machine will not pass the Apple Hardware Test.
A few other wrinkles – there are inconsistent reports on the Web of owners having success with some Western Digital Caviar Black drives (as well as failures with the same model) and with a Seagate Barracuda XL 3 TB drive. SSDs installed in the main drive bay likewise seem to be inconsistent, some OCZ Vertex 3 models and Intel 510 SSD models have been reported to work without issue. What we don’t know is how the drives that worked differ from those that did not – possibly the firmware revision of the drive?
If you are installing a non-compatible hard drive, one workaround for the fan speed issue is to manually control the fans with software such as hddfancontrol or SMC FanControl This requires monitoring the temperature, and adjusting the RPM of the fans manually.
The iMac has one hard drive bay, and one bay for a SSD drive. If the iMac is ordered from Apple without a factory SSD, then it will not have the mounting bracket or SATA cables for mounting the secondary SSD. Adding the SSD to the iMac as a second drive is not for the faint of heart, it is a difficult procedure that takes a lot of time, and risks voiding the Apple warranty. Here is a site with iMac teardown instructions. Some people are using Velcro as an alternative to the Apple SSD mounting bracket. Another challenge is to source the SATA data and power cables that will fit into the tight confines of the case.
We’ll be updating this post as more information comes out.