Hi Mhahn,
Thank you for your direction!
When I connect this broken Edison to my Ubuntu server through the MINI BOARD, there is the /dev/ttyUSB0 there! (I also tried to connect it through Arduino extension, but no respond.)
I used "screen" to connect the serial port with "$sudo screen /dev/ttyUSB0 115200", but no respond and the Ubuntu terminal just stuck there.
In this test I connected 2 micro USB cables on both USB ports to ensure the power supply.
I also tried with single cable connected, still no respond even I can see the /dev/ttyUSB0.
BTW, there is no /dev/sdbX under the /dev folder with my broken board connected. According to the official recover and flash steps, Recovering Intel Edison If Password Is UnknownFlashing Edison (wired) - Linux it seems Edison must be mounted as a flash before any recovery and flash operation.
I asked my workmate here in labs who programmed on Edison in the past 6 months, but they never met this kind of question.
Any way to save it?
Thank you for your patience and kindness.