Sadly there’s a lot of miss-information still out there!
So let’s see if we can clarify things. The 2011 systems use a custom drive from the HD vendors that supply Apple. These drives have the internal drive thermal sensor wired to the spare wire within the SATA power connector.
The older 2010 systems leverage a header pinout to access the internal thermal sensor of the HDD. And the 2009 and older use a glued on sensor as the HD vendors didn’t offer internal sensors back then.
So what is this thermal sensor? What is it for?
This sensor was used during the manufacturing of the drives so a hot running drive would be pulled as not meeting the specs the maker was contracted to meet. It’s a simple NPN transistor monitoring the power going through How do you get a temperature sensor from a transistor? The biggest issue is the range of the sensor so compensation and an amplifier is often added to properly scale the sensor so it is accurate.
So while you can use the Apple optical drive NPN transistor you may not get the accuracy you want. At the time we where winging it as we hadn’t gotten access to the schematics to review the circuit, today we know better! So I don’t recommend using the Apple optical drive sensor as its not calibrated where the OWC sensor is!
OK, whats the story with the Y connector? All you are doing is shorting out the sensor line so yes SMC is happy as its getting a signal, the only problem its a useless signal! I love people who are not skilled in electronics discover a simple solution without understanding the ramifications! You really don’t want to mislead SMC as you’ll cook your system. Placing a gun at your head is a bad idea too! Don’t do this nonsense!
What also confuses things is if you have a SSD only system Apple shorts the logic board header for the HDD connection so the SSD’s SMART services is accessed. If you have a dual drive setup you need to enable it as the polling of the HDD SATA port is more frequent! The SMART polling is every 5 or so minutes unlike the SATA connection which is constant.
What about newer systems how do they work? The 2012 & 2013 systems still require the OWC sensor as Apple did not update the firmware with the newer version of SMART Apple peddled to the ANSI/EIA groups SMART consortium. It took some effort on Apples part to get the needed change so the SMART thermal sensor IRQ method could be also polled directly. So what is IRQ Interrupt request which causes the computer to go into a wait state for the request loosing processing time for the interruption. If you remember I explained the Apple SSD uses SMART in the 2011 systems and its poll is overly long which is not as big a deal with an SSD (at the time) but would have been an issue with HDD’s.. Todays denser SSD’s get warm and the 2TB and bigger drives can get quite toasty!
So let’s move on the 2014 systems and newer: If you’ve maintained the OS with macOS Mojave or newer your systems firmware would have been updated with the newer SMART services. But there is still the drive! It needs to also be running the newer version too! You then don’t need the OWC sensor.
So! For your system follow this guide iMac Intel 27" EMC 2390 Hard Drive Replacement and getting the kit iMac Intel 21.5" and 27" (Late 2009-Mid 2010) SSD Upgrade Kit will get you everything you need. And sadly you need the sensor!
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