@Kubla Cant - MTP instead of USB HDD mode
Yeah, this was really off-pi... um, off-putting. I have an HTC One X with fixed internal storage: no SD card.
I managed to work out (with difficulty) how to make Kubuntu on my PC communicate using MTP, but I'm stuffed when it comes to the car: I used to be able to connect with USB and play podcasts, music, etc. from the phone. Now the car headunit just reports "Media Error".