Re 500GB HD
Problem is the power load drawn from the USB. I have solved the problem using the USB port and power socket in the Armrest.
I have a USB cable with 2 USB A (large flat plugs) at one end and a USB mini at the other end. The second USB A is only to supply power to the HD etc.
http://www.ebay.co.uk/itm/USB-DUAL-P...item2c67353a2a
Get a Car USB adapter to feed the second USB plug.
http://www.ebay.co.uk/itm/Griffin-Ca...item2ec8ef55df
This has worked well for over a year now.