I have a usb mouse, printer, ext. HD split in to 2 partitions (usb0 & usb1) a usb controled relay box, and the 8900 cable........did I forget something??????
Most of that won't show up as a USB serial device. I'm still looking for you to say what kind of cable and adapter you're using. Perhaps you're using something that has a USB connector on one end and the 8900 connector on the other?
If you unplug your 8900, do this:
ls /dev/ttyUSB*
then plug it back in and run that again, you should be able to see which device pops up when you plugged in your cable. That is the one you should use.
The other one is probably your relay box. Note that the devices are assigned in order. So, if you plug them in in a different order tomorrow, you'll need to change what you tell chirp to use.