Fat vs. Fat32
The file system used by all TomTom GPS that accept storage media cards is FAT. FAT is short for File Allocation Table, which itself is one of the key elements in the FAT file system. The file system keeps track of where each file is stored on the card. A FAT file system is applied by formatting the memory card. During the formatting process, the components of the file system - File Allocation Table, Master Boot Record and more - are added to the card's data storage area, in effect preparing the card to receive data from your GPS unit. There are 2 versions of the FAT system that are used. There is FAT16 and FAT32. For capacities ranging between 16MB and 2GB, GPS expect the card to contain a FAT16 file system. It's called FAT16 because it uses 16-bit numbers to reference locations where data is stored on the card. All current TomTom GPS are FAT16 compatible.
Unfortunately FAT16 simply wasn't designed to track files on media larger than 2.2GB. For any Flash memory card greater than 2.2GB, the FAT32 file system must be applied to take advantage of the full capacity of the card. FAT32 uses 32-bit numbers to point to where the bits and pieces of a file are stored. As a result, FAT32 supports media up to 2 Terabytes (TB) in size.
It's important to understand that any card with a capacity of more than 2GB will be only fully compatible with GPS that explicitly support FAT32 (some don't), and that your computer may choke if you try to copy data from a greater than 2GB card that has been formatted FAT16 (which is possible, although you'll only have access to a maximum of about 2GB of the card's actual capacity).
All memory cards up to and including 2GB in capacity are pre-formatted with a FAT16 file system. Cards 4GB and higher will have a pre-installed FAT32 file system. This helps the host GPS to detect the proper file system required for that specific card when formatted within the GPS.