I encountered a strange problem today trying to connect to a Dash through USB on Linux. I have a less usual Linux platform, with both USB 2.0 and USB 3.0 controllers and ports, and the distro is Linux Mint (so, Ubuntu). When I attached the Dash, Linux would not recognize the device, complaining that it couldn’t enumerate that USB device. With more precise error messages put into a search engine, similar complaints appear in Arduino forums, even a teensy forum, with no definite and specific solution, as there could be multiple reasons for failure.
Ultimately, I tracked this down to a problem with the ehci-hcd driver module. We had been plugging the cable to the Dash into a USB 2.0 port. HOWEVER, when we plug into a USB 3.0 port, Linux uses the xhci-hcd module, and it correctly installs the cdc-acm module which builds /dev/ttyACM0 properly. I don’t know why this should be so.