Dead after plugging in external 3.5inch hard disk

Hi

A few days back I plugged in an external 2TB 3.5ïnch hard disk on my powered ON iMac (27inch, Late 2014 EMC 2806 A1419). As soon as the drive was plugged in the screen went black. For a moment I thought screen went to sleep but pressing keys did not wake it up. This was the first time I had used this specific SATA/USB casing with a 3.5 inch hard disk.

Exploring further I found that I was not able to power up the system. No beep, no fan. Removed the LCD and saw that only the 1st diagnostic LED was ON with the power plugged in but pressing the power button caused the 1st LED to momentarily turn off and turn on again. All other LEDs remained off.

My iMac has 820-4652-A motherboard.

Removed the PSU and Motherboard from the housing.

PSU voltage are as follows:

12V on Pin Nos. 4, 5, 6, 10, 11, 12

Ground on Pin Nos 1, 2, 3, 7, 8, 9

On the motherboard no physical damage could be seen on any of the components (viewed under magnification). No cracked, broken, or burnt components seen.

I tried using AI to help diagnose and shortlist possible components to test. Unfortunately the naming IDs that AI is using does not match the BoardView or the board schematics that I have downloaded. For instance the fuse identifiers F7000, F7100, F8400 etc. According to AI Apple has changed identifiers and there are no markings on the motherboard silkscreen.

I was also not able to find any information on the web that related to the component identifiers or precise locations of these fuses.

I would really appreciate if anyone can help me with the troubleshooting my iMac board.

Thank you

Sajid