1. Applying 5 volts to Vin is too low a voltage for the on-board +5 vdc voltage regulator to operate correctly, it should be in the range of 7 to 12 vdc or so. Applying +5vdc to the 5V pin will work as you are then bypassing the on-board voltage regulator to power all the stuff on the module directly from your external regulated +5vdc power supply.
2. Yes it is safe, it's not about being smart it's the fact that the Vin voltage only goes one place, to the input of the on-board +5vdc voltage regulator which will then power all the stuff on the module.
3. If the board will not work on USB only power and you have tried more then one USB connector on your PC then you most likely have a bad (open) diode (D1) on your module and replacing it should correct the situation.
1. What you say makes sense but when I connect the board to the 12V(1A) source Windows XP never recognizes the device. When I go back to the 5V(3A) connection, wXP then recognizes it.
2. Ahhh okay. I learned something new today!
3. Yes, I tried more than one USB port. And actually, it makes sense that I killed the diode because before the blue power LED was very weak when only connected to USB. Now, it doesn't light up at all! So looking at the schematic here:http://arduino.cc/en/uploads/Main/ArduinoNano30Schematic.pdf
I think I have to replace part MBR0520. Digi-key has so many versions of this part:http://www.digikey.com/scripts/dksearch/dksus.dll?vendor=0&keywords=MBR0520
How do I know which one to choose?
Is this(see attachment) the part,D1, on the bottom of the board(highlighted in RED) that I have to replace?