4MOT Error Preventing High Sierra Install

27” iMac Mid-2011

• CPU 2.70 GHz
• SMC Version 1.72f2
• OS – El Capitan

 Problem:  When I try and install either Sierra or High Sierra, I get the following error during the installation:  “macOS could not be installed on your computer.  An error occurred installing macOS.  Use Apple Diagnostics to check your MAC hardware…”

When I run AHT, it gives me the following error code: 4MOT/4/40000003: HDD-1217. From what I understand, this is a fan motor error for the HDD.

Question:  I do not believe I have an HDD Fan problem.  See Background below.  Is there a way to clear the 4MOT error?  I’ve reset the SMC and NVRAM and the error remains. I believe the 4MOT error is preventing me from installing High Sierra.

Background:  My iMac was restarting on its own periodically.  I ran AHT and received the above 4MOT error. I noticed the iMac seemed to be running hot.  I installed MACS Fan Control.  When it Auto Mode, the HDD Fan would run at a high speed 5,000+ rpm which indicated a possible problem with the HDD.  After looking at various forums, I installed an OWC 2TB HDD kit that includes a temperature sensor.

I wanted to do a clean install of High Sierra, the last OS you can install on a Mid-2011 iMac.  I received the Installation Error above.  AHT indicated I still had the 4MOT error. After several times trying to install High Sierra,  I restored my iMac from Time Machine which has El Capitan.  The restore went fine and the iMac runs fine. I checked the MACS Fan Control and the HDD fan in Auto Mode runs at a low speed ~1,200 RPM.  The iMac is noticeably cooler with the new HDD.  With MACS Fan Control, I can manually adjust the speed of the HDD Fan.  I can hear and feel the fan when it is in high mode.  To me the HDD fan seems to be working correctly.

I then tried to install High Sierra as an update to El Capitan rather than a clean install.  I got the same installation error.

I believe the 4MOT error is preventing me from installing High Sierra.   I called Apple Support.  They had me do an SMC Reset and NVRAM Reset.  Neither cleared the error code.  Their solution was for me to take the iMac to an authorized service company.

Questions:

1.  Is there another possible cause as to why High Sierra will not install other than the 4MOT error?

2.  Is there a way to clear out the 4MOT error or get around the error in so I can install High Sierra?

3.  Is the only solution to either replace the HDD fan myself (which looks to be much more involved than replacing the HDD) or take the iMac to an authorized service company?

Any suggestions appreciated.