So, I tested a few uses on the USB port.
- Standard USB "thumb drive" (only 8GB) --Worked, as expected.
- Unpowered USB hub with two 8GB thumb drives - "Device unsupported." (but it was getting power)
- Single USB powered hard drive (500GB) - Device not attached (but it was getting power.)
- AC powered external USB hard drive (also 500GB) - Worked as expected. (inital read and load times were longer than normal, but that's expected as well.)
I don't have a powered USB hub to test, but I believe it would also fail.
I do not have a dual USB powered hardrive, but I believe it would work, if you know which is for power and which is for data. Plug the data cable into the car's USB, and the power cable into a USB car charger into the socket.