Problems opening serial connection to powered Arduino

I have an arduino based device (using Arduino Nanos) that are powered by external 12V power. It seems that in order to get my Mac to recognize the USB being connected the power must be off, although it can be turned on later and the connection is maintained. Is there some way to get a computer to notice the USB being connected on an Arduino that is already powered up?

I have only tried this using a USB-TTL cable (also known as an FTDI cable). The PC will recognize the cable without anything connected at the other end.

Just to be clear, I have NOT TRIED this using the regular Arduino USB connection.

You need to be careful how you connect to the Arduino so you don't cause it to reset - if that matters to you.

...R

Hi,
Perhaps I'm lucky but no problems using a mac! You may have to try differnt ports, but in general I fine it OK.

Regards

Mel.

What variety of Arduino did you have success with?

To be specific, I'm using Arduino nano's for this project, which use an FTDI chip for the USB interface. I'm wondering if perhaps the FTDI chip already being powered when the USB is connected is what prevents the computer from noticing the newly connected device.