Comment 2 for bug 236021

One way to address this is to add vfat to initramfs-tools/hook-functions

--- hook-functions 2008-05-30 14:04:09.000000000 +0100
+++ hook-functions.ago 2008-05-30 14:04:41.000000000 +0100
@@ -207,7 +207,7 @@
        base)
                for x in ehci-hcd ohci-hcd uhci-hcd usbhid usb-storage ext2 \
                ext3 isofs jfs nfs reiserfs udf xfs af_packet atkbd i8042 \
- virtio_pci; do
+ virtio_pci vfat; do
                        manual_add_modules "${x}"
                done
        ;;

Another way is to add a hook (/usr/share/initramfs-tools/hooks/vfat) via lupin-support that makes sure that vfat gets into the initrd.

=======/usr/share/initramfs-tools/hooks/vfat===========
#! /bin/sh -e

PREREQ=""

prereqs () {
    echo "$PREREQ"
}

case $1 in
prereqs)
    prereqs
    exit 0
    ;;
esac

. /usr/share/initramfs-tools/hook-functions

manual_add_modules vfat

exit 0