Sounds like you have an HDD unit fitted to the auxiliary. My ML MM also has a 60 gig one, but it's an iPod ...comes up on the screen as a secondary CD drive with the Lexus iPod integration unit. I haven't tried to run an additional drive through the Aux port under the central armrest as well as the iPod which is now in the glovebox.
The onboard disc changer is a six slot for DVD or CD. The Nav unit runs off a separate DVD unit, not a hard drive.
Hi, Dave are those wheels on your is350 the Toms vp-8 19 inch as shown on your site