The Arduino Nano Rev. 3 board (initially a product of US company Gravitech) with FT232RL FTDI USB to Serial communication chip may have a significant issue that makes some Arduino boards not readable from the computer.
The reason? The FTDI chip has ungrounded pin 26 which turns the FT232 into Test Mode. Such board is not visible as USB device, can’t be programmed via USB and can’t be operated in usual way.
It is apparently a bad design, a short look into Arduino Nano Rev. 3 shows the unconnected pin 26.
Arduino Nano Rev. 3 schematic – pin 26 unconnected!
It can be fixed by a simple solder bridge between pins 25 and 25 on the FT232RL. This fix (modification) sounds to be pretty simple and is – but is quite complicated to do due to extremely small space between pins, even with best tools and powerful magnifier. You will see how big is a tenth of a millimetre!
Arduino Nano Rev. 3 fix.
The Gravitech boards have outstanding compatibility with all operating systems but the boards made in 2009-2010 years may suffer from this disease.