This is / was implemented as a custom protocol to transfer files. It doesn't work out of the box with all programs on any system after the iPod is plugged in - it needs a program (consider it a driver) to be installed on the host computer for it to work. It only works with the programs that speak the protocol - installing the program does make it available to all programs on the host. This is quite different from making the iPod behave like an external hard drive.