The USB system has limits that CANNOT be overridden. While some people have reportedly gotten 32gb drives to work, larger USB drives are NOT supported. There's also a limit on the number of folders, number of files per folder, and overall number of files that the system will recognize. These are noted in your
Owners Manual. Also the USB has to be formatted as FAT16 or FAT32 as it will not recognize the NTFS format.
You'll find an ipod will give you more flexibility, but in all honesty most people just replace the head unit with a decent off the shelf radio with more functionality and better sound.