iMac Intel 27" EMC 2309 (Late 2009, Core 2 Duo 3.06 or 3.33 GHz) ID iMac10,1, EMC 2374 (Late 2009, Core i5 2.66 GHz or Core i7 2.8 GHz) ID iMac11,1

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HDD proximity sensor not returning data

I've been struggling with my 27" iMac (late 2009) for a while:

Symptoms: LED backlight randomly turns black in intervals that increase with temperature. The rest of the Mac and graphics card are working normally, an external monitor (via MiniDisplayPort) is working fine. My hypothesis is display panel is shutting down because it thinks it's overheating.

So far, I've replaced the VSync cable and LED driver board. The Mac has an aftermarket SSD replacement with the OWC temp sensor solution installed.

After almost giving up (I thought the display panel itself was faulty), I discovered I could circumvent the problem with a software solution (like iStat Menu and similar) which replaces system temp management with its own solution (automatic fan setting). Display now stays on at full brightness.

In iStat Menu readouts, I can see that HDD proximity sensor is not returning data. However HDD temp sensor IS!

My question: what and where is the HDD proximity sensor? Is that the one inside the old HDD, which I replaced with the OWC solution? Does it mean the OWC is faulty or badly installed? Or is it another part entirely? If so, how come I can read HDD temp data but not HDD proximity?

Update (11/02/2017)

I'm now thinking: either what iStat Menu is reading is actually internal SSD SMART temp data and OWC solution is dead. OR iStat Menu falsely expects HDD proximity sensor, which this model does not have, OWC solution registers as HDD temp and is working fine.

Answered! View the answer I have this problem too

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I would give a different monitoring app a try to compare the mis-temp. Give TG Pro a try


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Thanks for your tip, Dan. I used TG Pro, as you suggested. Using this, I found out that the HDD temp data in iStat is indeed the SSD internal SMART sensor. iStat Menus shows this as generic "HDD temp" sensor. TG Pro correctly lets you include or exclude SMART data. Conclusion is that OWC solution is either faulty or badly installed, and this MAY be the culprit for LED backlight shutting down randomly. I contacted OWC and customer support was quite good. Representative is sending me a new sensor free of charge. Although the rep wasn't knowledgable enough to help me with my problem, but that's too much to ask. At least I get a new sensor, no questions asked. Thanks again Dan.

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Happy it worked out!


Hello Joseph,

I’m curious: has the new sensor solved your issue?

I have the exact same iMac and noticed the exact same thing looking at iStat after an OWC. SSD upgrade with the thermal sensor kit. SMART sensor gives Me a SSD temperature reading while the hard drive proximity sensor gives no reading at all.

From what I understand, I would have expected the fans to rev up at full speed to protect the drive if, for any reason, the HDD proximity sensor was inoperative.

Thank you,



I ended up giving up on this iMac for another reason, so I never installed the replacement sent by OWC.

The OWC sensor, whether working or not, somehow did prevent the fans from revving up under system temp management. Even though there appeared to be no temp data coming from it.

But the display turning black seems to be a very complex issue. It happened less frequently after the upgrade, if I remember right. I now think that it could be caused by faulty LEDs themselves, which causes it to draw more current. The system shuts down the display as a protection. Maybe the temperature is only a secondary effect.

It was not cost effective to replace the display panel, unfortunately, so I had to let this machine go.




Hello Joseph,

I contacted OWC for a replacement sensor. Great customer service indeed. I should receive the sensor in a few days and will report back to tell if I get a reading out of the new one. Could be helpful for others.

As for the black screen, I too had what could be a similar issue after my drive upgrade. Graphic artifacts and the screen momentarily turning black from time to time for a second or two.

In my case, it was the LED female socket that comes out of the board that was misaligned, sticking out with an angle and putting pressure on the Male connector when closed . The two weld-on legs that secured the socket to the board had unwelded due to too much force applied on the connector. I ended up gluing it back in position on the board with hot glue, which is what I had in hand at the time and that I believe is safe for electronics. Don't know if it will hold on over time with the internal heat, but so far, it does the trick and fixed my display issues.

Thank you for your reply!


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Joseph Spackenberger will be eternally grateful.
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