As far as I remember the disc is formatted in a standard way because B&O hasn't invented something new. I think it's the ext-Fileformat that is used by Linux. Most Harddiscrecorders use the ext-Format because it's very compatible and secure.
The Best way to upgrade is rebuilding the disk from the HDR and connecting it to a PC. Then clone the disk. Second you have to clone the disk to a new one. If the new disk is bigger than the old one you also have to rebuild the the tables so you can use the full size of the disk.
The best way for all this is to use the terminal in Linux or Mac OS
But if you're not that into computers you better contact a dealer he can do it for you as you can read here.
By the way FAT is crap because it doesn't support files over 4GB so it can't be used for recording movies