Today I purchased a Freecom Tough Drive 120GB mobile USB hard drive. Its USB powered and guess what... It works with the car stereo.
I have been trying a number of different USB devices with my Kenwood head unit to see if they would work, like the Nokia N95. You select the phone to Mass storage and the head unit reads it as if it was a USB stick.
I have also wired up the audio/visual lead to the screen in the car so you can use the phone as normal but view it on a large screen.
The question now is how many music files the head unit can see, I am in the process of copying 47GB's worth of music to the drive to see if it will ok but as I am not running windows it should be fine. Navigating will be a problem, and there will be a lot of track skipping.
Not bad for 54 quid...
On another note, due to the weight of the DVD player the bit of plastic that sits under the glove box gave way this week. Need to figure out a way of re-fitting the DVD player and make it look pucker!