Hi,

quoted

vfat module was not loaded before, it got loaded only after I issued
the mount command. We can see this info in the udev log. Moreover
there was no entry for the usbstick partition in /etc/fstab. So
someone kernel/userspace takes the decision that vfat.ko should be
used for managing fat32 partitions. I was trying to identify this
component.

'mount' itself was responsible for guessing the vfat driver. Running
mount with -v gave the following output

# mount -v /dev/sdb1 /mnt/usb
mount: you didn't specify a filesystem type for /dev/sdb1
       I will try type vfat
/dev/sdb1 on /mnt/usb type vfat (rw)


-- 
Thanks
Sudheer