The partition is created on the embedded linux target like this

#   n       - new
#   p       - partition
#   3       - partition 3
#   66      - starting cylinder
# <blank> - maximum size for the ending cylinder
#   t       - set file system type
#   3       - partition 3
#   c       - set to windows vfat
#   w       - write partition table and exit
echo -e "n\np\n3\n66\n\nt\n3\nc\nw" | fdisk /dev/mmcblk0

The file system is then formatted on the embedded linux target as MS-DOS like this

#   -n volume-name
#   -F FAT-size
mkfs.vfat -n DB -F 32 /dev/mmcblk0p3

A linux host can mount and access files in mmcblk0p3 without issue. Why can't windows?

Edit: Although the default number of FATS is 2 I tried adding

-f 2 [number-of-FATs]

since this is actually being done by busybox on an embedded platform but this didn't help. I understand the Linux MS-DOS file system does not support more than 2 FATs but there are only 2 on this target (the boot is also FAT which is visible), along with and EXT3 (on p2) for the root file system.