Here you see two diodes one is 5 volts second is 12 volts .In SATA power connector 2 kinds of voltage is available one is 5 volts and second is 12 volts. If we plug connector in right direction the current flows in right direction and diode allows the current to go into the circuit . You also say it p-n junction . One is p region and second is n region . The 5 volts diode allows the current under 5 volts . If the volts exceeds to 5 then it burned and switch off the power supply if you are using switching power supply .

Why diodes are burnt ?
When we plug the connector in wrong direction a 12 volts passes to the 5 volts diode and it burnt.
And power supply is switch off due to short circuit.
The circuit is burn when we connect wrong power into the connector of the HDD.

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Banned user

Hello there everyone. A good way to start will be first checking the pinout and then comparing how the pins actually get connected when reversed.GENERALLY PN Diodes as semiconductors blow off when the reverse voltages crosses the specified threshold(reverse breakdown).

Here sir for your further reference. https://en.wikipedia.org/wiki/Breakdown_voltage

Rgds
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