Is there a lower level log of USB activity than Console.app or system.log?
You need to install the usbutils commands via macport (sorry, I don't use brew) 

sudo port install usbutils


then, for a first check, try the following command: 

lsusb


You should see a list of your USB devices. It might be hard for you to identify your device. A good way to filter all your devices is to remove every of them except your USB/serial adapter. 

If lsusb returns nothing. Your device is dead. 

If my memory is good, the name of an USB-serial adapter printed ends by: 

 USB-Serial (UART) IC