In both cases the SMC system tends to go into frenzy and drive you mad with a loud fan. Sometimes reusing the external sensor is not possible or, as in more recent generations of iMacs, replacing the original hard disk breaks the system’s temperature monitoring. The SMC collects data from external sensors or by reading the internal hard disk temperature in a proprietary manner. Fan management in Apple systems relies on temperatures reported by the SMC system. IMac users who tweak their systems using 3rd party hard disks or solid state disks, are often surprised by an unpleasant aftermath: an annoyingly noisy hard disk fan.