Last summer I acquired an Apple iMac 27" late 2009 model EMC 2309. It was being thrown out because it was no longer able to boot(would not pass the grey screen), and the owner decided to buy a newer one. The computer's death was not sudden, but rather became gradually worse over time, where more and more frequently the screen would suddenly go black, requiring reboot. However it was not until the previous owner tried to install High Sierra that the iMac died completely. I heard through the grapevine that this is a common problem with that vintage, and it was explained as the result of hardware failure in the graphics card, due to general old age...
I was not really satisfied with that explanation, and it didn't really seam right to me, so I decided to downgrade to Snow Leopard, and viola it booted and functioned normally. Since then I have upgraded to Mavericks and everything seems to be running fine. Safari is stuck on 7.0, but it turns out it will run Firefox 66.0, and many other apps that still support Mavericks.
I don't know what is more troubling, that someone would try to upgrade that old hardware to 10.13, or that the upgrade was done through the AppStore. What I am wondering is, since there is clearly nothing wrong with the hardware, why would Apple post an upgrade in the AppStore that will totally not work on your computer? Shouldn't there be some sort of supported-hardware list?
Is this something special to this model? This computer? I suspect it's a similar story for other ~10 year old Apples.
Things seam to be running smoothly now. Should I risk going up to Yosemite? Remember, it was not until Sierra that problems really started occurring. What features added in El Capitan and Sierra could be the cause of this problem?
I did some googling and found some similar reports that relate to my situation going back to 2012. Some of the older posts don't have clear solutions, or can be linked to loose connections: 2012-2018-macrumors: iMac screen goes black. must hard reboot. HELP
Here's another unsolved, although some later posters give strange solutions including ones involving a second monitor: 2014-2015-apple.com: iMac Screen Goes Black Without Reason
ifixit blames the LED driver board(this one also points to a possible graphics card issues): 2017-ifixit: Screen randomly goes black 27" iMac late 2009
Here are 2 more recent reports; oddly in both cases it appears that when the screen goes black, the iMac itself is still on, and can still be operated along with an external display. This is interesting because for the iMac in this question, the fans were noted to remain running after the screen went blank, although no external display was used to test:
It seems plausible that these last 2 unsolved cases could be experiencing the same issue as my now rehabilitated iMac running Mavericks. I am not sure what OS they were running though, as neither says. By the dates of each post I suspect Sierra or newer.
Mac OS X released 10.6 Snow Leopard August 28, 2009 10.7 Lion July 20, 2011 10.8 Mountain Lion July 25, 2012 10.9 Mavericks October 22, 2013 10.10 Yosemite October 16, 2014 10.11 El Capitan September 30, 2015 10.12 Sierra September 20, 2016 10.13 High Sierra September 25, 2017 10.14 Mojave September 24, 2018