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Model A1311 / Mid 2011 / 2.5 & 2.7 GHz Core i5 or 2.8 GHz Core i7 Processor

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Why does my iMac turn to a Gray Screen while installing OS El Capitan?

I replaced my hard drive with a 2T HDD also replaced SATA power cable. When I turned on my iMac, I held down the option key and selected my external 1T HDD in a docking station, which has El Capitan installed on it as a boot-able drive. I saw the apple logo appear with a generating progress bar and then it switched to a blank Gray Screen for an hour. I could hear the fans blowing like crazy and my external boot drive was not blinking progress anymore. I think it is a hard ware issue but not sure whether I should replace GPU or something else. I would like to keep this model iMac for a little bit longer because I can’t afford to buy another one. I greatly appreciate any insight or resolution for this.

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With the external drive disconnected with a USB keyboard plugged in, restart your system and press the D key to enter into the onboard diagnostics did you get any errors?

Reference: Mac startup key combinations

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@danj Dan, yes. Error note: 4SNS/1/40000000: TH00-9.000

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Your system leverages the original HDD’s internal thermal sensor. Apple moved from a stuck on sensor to using the drives diagnostic thermal sensor each drive maker had in there drives for QC, The 2011 systems went one step farther! Apple had a custom sensor setup just for their drives. The error 4SNS/1/40000000: TH00 is telling me the sensor is not present in your current drive.

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To get around this you need to install a replacement sensor when you replace the original drive with either a HDD or SSD OWC In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade

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I will attach the OWC In-line Digital Thermal Sensor to my Upgraded Hard Drive and see if that alone will resolve the error. Hopefully my 2011 iMac will boot normally without issue. I will let you know soon.

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@danj After attaching sensor as previously noted. I rebooted and got gray screen but this time with a flashing question mark folder. So I did another diagnostic test and no errors. It read no problem. Verified HDD power cable connection to logic board. Checked cables for damages or connection issues. All looks clean and secure. This time iMac was quiet no loud fan noise. Attempted to install OS but got progress bar below apple logo as it generated more than half way through then screen quickly switched from black to gray and stayed there.

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@learning2grow - Thats a good sign!

Then we have to prep the drive now, restart holding the Option - Command and R keys to enter into Internet recovery to first format your drive to GUID and with a Journaled file system, then install the latest macOS your system can support. OR, from a second mac system create a bootable drive with the required OS you system can support I recommend sticking with Sierra on your system.

Reference: Mac startup key combinations

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@danj Okay I will do the Internet recovery format drive process next and see how my iMac responds.

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@danj I restarted holding the Option - Command and R keys to enter Internet recovery. All going well. Got the spinning globe with progress bar below. It finished. Then apple logo appeared with progress bar generated more than half way through and that is when the screen quickly flickers into a blank gray screen and remains there. It look like it was trying to switch over but something is blocking it from going to the next phase. It will not switch over to a screen where I can format the drive. And when I restart my iMac normally then I keep getting a flashing folder with a question mark.

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Sunflower De will be eternally grateful.
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