The D3 has only one disc slot, that's to what I was refering.
Also, the D3 is wired to both 12V and ACC, so it's getting power all the time. There's no need for it to store in ROM, although it may store (and hopefully does store) some settings in ROM, such as EQ settings, presets, background images.
The map data is read off of the DVD disc and stored in the system's RAM, though.