I don't think it will change anything, but could you measure the voltages at each pin of U3 (near V3)? I don't know what this chip does, but I'm wondering if it is a power supply monitor.

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A backup a day keeps DR away.

Sure, if you have the $$$$$

_________________
A backup a day keeps DR away.

U3 Pins voltage readings (start from white arrow) pin 1-6 anti-clockwise order:

1=1.79
2=0.5mv
3=3.06
4=3.17
5=0.5mv
6=3.28

ICBW, but U3 appears to be a dual voltage monitor with individual reset outputs on pins 3 and 4. Pins 1 and 6 appear to be the monitored voltages, namely 1.8V and 3.3V. ISTM that the outputs are OK.

_________________
A backup a day keeps DR away.

I got another working 64GB unit, excat same Vertex 4 product.
Might worth open it and compare at certain point that can lead us to solution?
Or the "CPU Panic mode / FW fault" will be the case no matter what we do?

I don't think that will help us.

You could experiment with the firmware test point and observe how your good drive behaves when this test point is shorted to ground at power-on. Then check whether the data are still intact. This will at least tell you whether or not the controller is brain dead.

You might also like to connect a USB-TTL serial adapter to the Tx/Rx UART pins and see whether there is any diagnostic output.

_________________
A backup a day keeps DR away.