Vivamort:
I need a micro controller with a USB slave and Host interface (i realise that is not on the Arduino) to act as a go between a PC USB port and a USB mass storage hard drive in both directions.
Most Arduino boards don't have an innate ability to act as USB hosts. However, there are USB Host shields (like this one or that one) to enable them to act as USB hosts. Also I think the 2011 Arduino ADK board has the Host/Slave interface as a built-in feature.