I’ve been reading this thread and the Y Spitter solution. I am 100% sure this is just grounding the temperature sense data pin 11 on the sata power plug to the hard drive. These sata power splitters and the connections on a standard ATX power supply tend to be be wired with 5 x 3 sets of pins bridged together. 3.3v , gnd, 5v, gnd, 12v.
The 3 pins 10, 11, 12 in one of these splitters are all grounded. Maybe grounding this temp sense pin 11 shuts the fan up. I wonder what apple do in the late 2012+ models when it is a blade only SSD only model with no standard drive.
My other reading implies that in the pre 2012 models there was a blanking jumper plug that was sometimes put in the unsued sata power port to keep the SMC happy with no drive present.
Standard sata power wiring.
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Sorry if this has been answered I just thought my thoughts may be useful to anyone stumbling across this tread like i did.