I seem to remember that Kenwood produced a 20GB "keg" for storing Mp3's in there ICE system.
With the proliferation of 2.5" USB-2 disk caddies, this would be an ideal way of carrying/updating your music/videos in your carputer.
These disk caddies can hold a BUS powered 2.5" laptop drive, drives are available upto 80 GB nowadays. So keep your system disk small and cheap, use the caddie for mass storage.
I'm gonna look out for one that can be docked easily to create such a device.
Audi S8 supercar
Modded Xbox 400GB HD
Rear 7" screen & navplus
Toyota Hiace Van
Shuttle PIII 866 256Mb 10GB OPUS powered
MediaCar & Mappoint 2004
Custom fit 7" TFT VGA TSK