2009 E350 40GB Command
I would guess that the remainder drive space is reserved for the linguatronic (all various languages in which it is available), Navigation Maps for your country / region (i.e. the whole of west Europe is combined as one lot!), Database for gracenote etc. It is unfortunate that the end-user cannot allocate the space, since I would think that not every region/country's maps and database would amount to 36 Gb. The developers probably took the "safe" route and sufficient space for future upgrades/updates of the Navigation section.
Perhaps there are tweakers out there that know how to re-allocate the partions?
Then you can also add a non-SDHC SD Card for extra storage. Most manufacturers make non-SDHC cards up to 2GB, but Transcend and some other manufacturers make 4GB non-SDHC cards that I've read works.
Hopefully this all will give you "enough" space to play around with...



