Arduino Ethernet and Usb2Serial does not connect to COM port

Hi guys,

I have a problems with connecting my arduino to my PC. I'll try to explain what happened.

I was using arduino ethernet with usb2serial to play with servo motor (single) MG996R. My setup was : arduino ethernet, usb2serial and servo connected to Pin9 and +5V and GND from arduino) all of them powered from USB. I was playing few days, everything worked as charm but I wasn't satisfied with motor behavior. I suspected that problem was because I powered motor from arduino and not from separate power source. I've try to connect external adapter (12V) to arduino jack connector to give them more power (adapter was 12V and 3A). The behaviour of motor didn't changed, so I suspected that arduino curcuit can't supply enough power to motor. Then I try to connect external power supply (adapter 7.5V 1A) to motor (+ and GND only) and signal cable was still connected to Arduino (Pin 9). I also connected GND from arduino to - of power adapter.

It was working until one moment when PC start to not recognize COM3 port (where it was before). Arduino is still working, servo is moving using last uploaded program but I can not see any more COM3 port. I've tried to change USB, didn't help - USB is still working (I've checked with usb drive). Arduino can be powered from external power source (12V) or from USB - it continues to work but I can not see COM3 port.

Maybe I fried usb2serial but I don't know even how. I haven't connected this other power source (7.5V) to arduino - only to servo.

Also I had a problems with USB when external power source (12V) was connected to arduino. I had to turn power, connect only USB and then I had to transfer a program.

Any ideas, suggestions? thanks in adavance

Also posted at http://arduino.stackexchange.com/questions/33156