I was having same problems as mentioned. New nano, drivers failed to load in windows.
Tried downloading new drivers, same result.
Unplugged the nano from the IO shield and everything worked correctly.
Supplied stable 6v to power IO shield, everything worked correctly.
So I tried to find a way to increase the priority of the "FT232 driver" on my linux, but cannot find how to do this. It's not described in the AN220 FTDI Drivers Installation Guide for Linuxand the documentAN107 FTDI Advanced Driver Optionshas a capter about "Changing the Driver Priority" but only for windows.
Step 2: Download a proper FT232R USB UART driver from the FTDI VCP driver website and extract it. But this time, you should download the manual version. Please pay attention to the OS version (32-bit or 64-bit).
In device manager it said that windows has no compatible drivers for this device. I installed drivers from the FTDI website and all it did was change the name in device manager from FT232R USB UART to USB Serial Port.