For  USB flash size >= 64GB here is revision code.

opkg update
opkg install bash
wget -O newexpand.sh https://www.dropbox.com/s/jnrwhrc0dhjynyp/newexpand.sh?dl=0 --no-check-certificate
/bin/bash newexpand.sh sdb

"newexpand.sh" code: 

if [ -n "$1" ]; then
device=$1
else
device="sda"
fi
driver=$device"1"
opkg update
opkg install e2fsprogs fdisk
if fdisk -l |grep /dev/$device > /dev/null; then
    echo "Start"
else
    echo "/dev/$device not found!"
    exit 0
fi
dd if=/dev/zero of=/dev/$device bs=1M count=10
(echo n; echo p; echo 1;  echo;  echo; echo w)  | fdisk /dev/$device
umount -f  /mnt/$driver
mkfs.ext4 /dev/$driver
mkdir -p /mnt/$driver
mount  /dev/$driver  /mnt/$driver
mkdir -p /tmp/cproot
mount --bind / /tmp/cproot
tar -C /tmp/cproot -cvf - . | tar -C /mnt/$driver  -xf -
umount /tmp/cproot
uci add fstab mount
uci set fstab.@mount[0].target=/
uci set fstab.@mount[0].device=/dev/$driver
uci set fstab.@mount[0].fstype=ext4
uci set fstab.@mount[0].enabled=1
uci set fstab.@mount[0].enabled_fsck=0
uci set fstab.@mount[0].options=rw,sync,noatime,nodiratime
uci commit
reboot