OWC's thermal sensor connects only to power - but sends data? I want to upgrade the hard drive in my iMac (21.5" Mid 2011) from 1TB to 3TB.

OWC sell an "in-line digital thermal sensor".  They describe the need for this:


  Unlike Prior Generations, Apple iMacs from late 2009 to current 2015 &amp; later utilize digital reporting and even custom firmware for SMC drive temperature reporting. OWC's Exclusive HDD Kits include our custom digital monitor that 'talks Apple SMC' and maintains proper temperature reporting and Apple Diagnostic compatibility allowing you to upgrade an existing 3.5" HDD with any 3.5" SATA HDD of your choice up to 8.0TB


Below is a picture of the sensor, which they sell here:



Given this "talks to the Apple SMC", I was expecting it to be connected to the SATA data connector.

But oddly, it isn't: it fits "in-line" to the SATA power connector.  (The two connectors on this assembly plug into the existing power connector in the iMac, and into the power connector on the hard drive... see OWC's video starting at about 4:13 for instructions.)

I've looked at the Wikipedia description of SATA power connectors but don't see any mention of this kind of signalling.  (Pin 11 is for "staggered spinup / activity", but from the description, this doesn't seem capable of passing information about temperature.)

How can this sensor communicate with the iMac when it is only plugged into power?
How can this sensor communicate with the iMac when it is only plugged into power?


Because it's not only plugged into power; look at the number of connectors on the logic board side compared to the number of leads on the drive side.

You are also working on the assumption that the power connector is "power out only."  This is not the case.  The SMC can read changes in expected voltages across a circuit as well as send/receive signaling embedded within the power