FAT16 without backup FAT.
Some programs e.g. SK Tools can format 64k clusters i.e. allow to format 4GB cards with FAT16. This may be non-standard but neither WinXP nor the x50v complain. My Kyocera camera accepts 1GB cards (16k clusters) but rejects FAT16-formatted cards using 32k (2GB) and 64k (4GB) clusters. It can use the 2GB card if I let the camera format the card. However that results in RAW format.
Regarding FAT16/32, see
Menneisyys' article.