FTDI (via usb) doesnt work with external power supply

Hi,

I am using a usb serial converter to read and write bytes to a device.
If I power the bbb with an usb cable, i can see /dev/ttyUSB0. But
if I power it with an external power supply (5V, 5A) it doesnt work.

Is there something special I have to care about?
I am using Ubuntu 13.10 on the board.

thx.

Make sure all the the power supplies are connected to the same power strip.

Gerald