USB and uart woes

My setup is:

uart - external GPS 38400 baud
USB0 - mavlink telemetry via 3DR radio 57400 baud
USB1 - mavlink telemetry via USB-TTL adaptor 57400 baud

my startup line is:

/home/pi/APMrover2.elf -A udp: -C /dev/ttyUSB0 -D /dev/ttyUSB1 -E /dev/ttyAMA0

The GPS works fine but it comes in MP as the second GPS so it must be serial4 ???

Based on the forum -C is serial1 57600 default & protocol in MP set to mavlink
and -D is serial 2 57600 default & protocol in MP set to mavlink

If I plug the 3DR radio into any USB socket then on boot is works. So I assume that the first device is sees is assigned USB0.

If I only plug the USB-TTL adaptor into any socket it works after boot.

BUT !!! If I have both plugged in and then boot - only one works (random). I can see that ttyUSB0 and USB1 exist and they are assigned to the devices.

What is going on ??

Hello there!

A very similar problem came up just a couple of days ago here.
I suggest taking a look! There’s also a thread that goes about -D switch. I guess that after reading through both of these threads all of your questions will be cleared. If not, feel free to ask!