Arduino Uno::: Mac OS X 10.7.2 Lion: Not able to Upload


I recently bought a new MacBook Pro and am not able to upload sketches to my Arduino Uno. The board works perfectly fine on my older mac running 10.5.8 Leopard, but not on this comp running 10.7.2. Whenever I try to upload a sketch I get the following error: Serial port '/dev/cu.usbmodemfd121' already in use. Try quiting any programs that may be using it.

I tried according to other posts to install this FTDI Serial Drivers:

However, this did not change anything. There is no cu.usbserial.* port available only this cu.usbmodemfd121, which says its in use despite the lack of other applications running.

I have tried hooking it up to another usb port and it is not connected to a hub. Any help would be greatly appreciated. Thank you.


FTDI Drivers will not hurt or help a Uno, since the Uno doesn't have the FTDI chip.

Try this, at a terminal: sudo mkdir /var/lock sudo chmod 777 /var/lock

You'll be asked to enter your password once.


I am running Mac OS X Lion 10.7.3 (11D50b) MBP 13-inch mid 2010 4GB ram and I just downloaded the arduino app from the getting started page

Hope this helps. I am in the middle of putting together a aquarium water change system. I would love to use the either-net shield to run as a web server to show current status of all the solenoid valves and the pump. water levels in the tanks and the water conditioner (a 55 gallon food grade drum. with heaters to keep the water at 80F and a aerator to out gas any extra gasses like ozone and chlorine.)

Hope this helps

Try using lsof to see what else has the serial port open:

lsof | grep usb

It might shed some light on the problem.

I had the same problem.

The suggestion

lsof | grep usb

didn’t report anything, however the suggestion for

sudo mkdir /var/lock
sudo chmod 777 /var/lock

worked perfectly.