Replacing the HDD in a recent-ish iMac without an apple-branded disk causes the system to floor the fan RPMs making for a TON of noise. This script manually overrides the HDD fan speed every ten seconds to quiet things down.

hdd_fanctl.sh

#!/bin/bash

#smartfancontrol.sh

#### enter your preferred values here ##################
speed_min=2000 #RPM
speed_max=5500 #RPM
temp_min=46 #degrees celsius
temp_max=60 #degrees celsius

########## Do not edit below this line ###################


while true; do
  echo 1 > /sys/devices/platform/applesmc.768/fan2_manual

  temp=$(/usr/sbin/smartctl -A /dev/sda | grep ^194 | awk '{print $10}')

  if [[ $temp -lt $temp_min ]]; then
    speed=$speed_min
  elif [[ $temp -gt $temp_max ]]; then
    speed=$speed_max
  else
    x=$(($speed_max-$speed_min))
    y=$(($(($temp-$temp_min))*100))
    z=$(($(($temp_max-$temp_min))*100))
    a=$(($x*$y/$z))
    speed=$(($a+$speed_min))
  fi

  echo $speed >/sys/devices/platform/applesmc.768/fan2_output

  printf '[ %i C | %i rpm ]\n' $temp $speed

  sleep 10
done