iMac 27" 2011 Late HDD Thermal Sensor mystery and fun facts
Long story short: there is no such thing as HDD thermal sensor on iMac 2011, there is a thermal trigger. Simply tie pin 11 on sata power port to ground and forget.
Long story long:
U5400 LM393 is a comparator.There is 1V reference voltage on pin 2 produced by voltage divider. By default pin 3 is pulled up to 1.5v so comparator does nothing with 3.3V pullup on pin 1. SMC receives 3.3v and thinks that HDD is burning down your iMac.
If voltage drops below 1.0v LM393 pulls 3v3 to ground and SMC receives logical 0 which is considered low temperature.
What does fancy 50$ OWC thing: it takes 12V or 5V (who cares, never had one)from Sata cable, turns it into 3V and uses LM35 or similar to output 0.5...1.5V to pin 11.
Why does Y-Sata cable solves the problem: most Y cables symply short pin11 to ground. (do not use such cables in Servers unless you want to kill your PSU :) )
Fun fact 1: pin 11 is usually used on server hard drives to regulate "safe" startup by simply delaying motor spinup thus reducing peak current on 12v PSU line.
Fun fact 2: there are Apple Plugs for imac 2009-2011 for SSD setups. On 2009 it may be shorted, on 2011 poor Apple Technician should remove the logicboard and connect it to the HDD Power socket(J4511) on board. So by design, if you upgrade to SSD, it is totally fine to disable temp. trigger. Min.rpm is more than enough to keep even beefy high density SSD cool.
Fun fact 3: weird fan behaviour is actually a compatibility sacrifice: