Mid 2010 Mac Mini Server (Macmini4,1 - Intel Core 2 Duo 2.66GHz) The server had two hard drives (one formatted for the OS and the other for storage). Replaced the OS hard drive with a Samsung SSD. The removed HDD had a wire coming off it (turns out it was a thermal sensor) that the replacement SSD did not have. Upon reassembly and restart (after install of OS on SDD) the Mac Mini Server cooling fan went on high and stayed there. Problem was eventually solved by removing the thermal sensor from the old HDD and reinstalling the sensor in the Mac Mini Server (connector to mother board, sensor just stuck on something inside the case). /edit : ps. the sensor was simply stuck to the outside of the old HDD; removing it was merely a matter of sliding a razor blade between the sensor and the side of the HDD.