The Nano (v3.0) has a nasty gotcha: it works fine when plugged into an already-running PC, but if the Nano is already connected when 
the PC is cold-booted (from power-off), the Nano won't be recognised; USB errors occur.
This is because of a floating test pin on the FTDI232RL chip.

The fix is very easy: bridge pins 25 and 26 of the FTDI232RL chip (on the reverse of the board) with a tiny blob of solder. 
More details are in this post:

arduino.cc/forum/index.php?topic=23025.0;wap2 
