There is a problem with the fans on the iMac (2012's and on) once a HDD is replaced with a non 'Apple' HDD, i.e. the Fans turn on to high speed when power is of or in sleep mode. I may have accidentally found a fix (all the reset procedures have not worked for me). System Preferences, Energy Saver: Computer sleep slide - NEVER Display sleep slide - NEVER tic HDD to sleep tic wake up for network... tic power nap... Then leave the computer on without any interaction until the screen goes into a "screen saver mode" (thats sounds a bit strange that because you have selected NEVER). And that fixed it for me! Been running like it should ever since. Strange.