We got this 27″ iMac from a client for hard drive replacement. The existing hard drive was completely gone and was making funny noises. Hard Drive replacement on an iMac is a big job as components are tightly packed inside and a lot of things need to be removed to access the hard drive. One more problem with iMac is that the hard drive has a built-in temperature sensor integrated in the chassis. If you replace iMac hard drive with an ordinary SATA drive then the system cannot sense the hard drive temperature and the cooling fans start to run at full speed. Therefore, an iMac drive should only be replaced with a compatible hard drive. We replaced the hard drive with a new one and installed latest Mac OS. Another happy client!