So I can't find any documentation about this on Apple's website so I'm thinking it's a bug, hopefully in software (filed with Apple this morning). When I have a USB-C to DP cord plugged in (going to 4K monitor) on the left side of the machine, the machine doesn't recognize an external USB drive when I plug it into the other port on the left, and occasionally when I disconnect the machine locks up. However when I plug it into a port on the right side the HDD is recognized right away. This behavior isn't limited to the left side, it also happens on the right when the USB-C to DP cable is plugged in over there. Anybody else having issues with a 4K stream disabling the other USB-C port running off the same TB3 controller?