MTS, there are two units out there that come close to what you are looking for.
The closest is SSI's Neo Car Jukebox (
http://www.ssiamerica.com/products/neo35), a trunk unit like the Phatbox. When used with Open Source firmware it can accept any capacity 3.5" hard drive. There is a voice module available, and a CD changer module that will let it interface and be controlled by Pioneer or Kenwood head units. (Otherwise you have to use a satellite control module.)
The Dension (
http://www.dension.com/main.htm) DMP3 is another trunk unit, which can take any capacity 3.5" hard drive, and supports nested folders. However, no voice module, and the only head unit it can directly interface with is Dension's own DH100ix, which is also a hard drive player/FM tuner (no cd), that can take any 2.5" hard drive (which itself is removable). The DMP3 can be connected to other head units, but then to control it you have to use a separate satellite controller. The Dension units appear to have been discontinued in the U.S.
The best thing about both of these systems is that the companies freely allow you to add your own hard drives without gouging you for "properietary" drives. I'm generally happy with my Music Keg, but I'd love to see a 2.0 version that used regular 3.5" drives.