iMac Unibody HDD Thermal Sensor Options
This forces the iMac to read the SMART temp sensor:
Open up the SSD, and solder a bridge (solder-blob) between pins 10, 11 and 12 of the SATA Power connector (the longer of the two)
Will upload picture tomorrow for easier reference.
1. SSDFanControl. Set to use SMART. Will vary fan speed based on SMART temp sensor.
3. This is not tested but should also work fine: Use Y power splitter and scrape old HDD board to use it as sensor. Glue or tape it on SSD/HDD and connect it to Y splitter. You might need to cut ground wire next to 5v.
4. fancy 2N3904 by @EstébanM [SC,US]