In an online blog (http://macte.ch/8eKVk), Other World Computing says Apple has made further restrictions on ways to upgrade the latest round of iMacs.

Here's what OWC has to say: "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).

"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.

"To add insult to injury, the latest iMac EFI Update 1.6 unleashed 6Gb/s speeds on two internal ports – and naturally, one of them is the proprietary, firmware-limited, 7200RPM main drive that can’t take advantage of those speeds anyway.

"Now this isn’t to say that our Turnkey Upgrade Program isn’t going to include the new model iMacs. The external eSATA port, or adding hard drives or SSDs in addition to the main hard drive are still perfectly viable and working options in our testing so far. But it isn’t looking good at the moment to have the option to upgrade or even replace the main 3.5″ hard drive as shipped from Apple."