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I have no knowledge of the HDD electronics but looking at the instructions for making the serial connection one thing worries me a bit. I see it is suggested to use 5V for powering that circuitry. This means that the serial signal logic levels to the HDD would also be 5V. I don't know what is the I/O voltage for the IC on the HDD controller board but these days they are usually 3.3V or less. If there isn't any protection on board then using 5V levels could potentially damage the I/O pins. Of course the input pins may be 5V tolerant or they may be other protection measures on the PCB for these pins. But if there isn't any protection and the pins aren't 5V tolerant this could be an issue. If needed this could be solved by powering the RS-232 IC with lower supply voltage. Another way to limit the voltage could be to use zener diodes and current limiting resistors. Not sure of the appropriate values as I don't know what king logic levels the HDD interface expects. Like I said I have no knowledge of the HDD electronics details but I just realized this potential issue as I was reading the topic and thought I'd mention it just in case.