Prompt [research]Another way to extend /data

In almost mobile ,the /data is too small.Can we use extend tf card instead data to get more space?
I try do something:
use a 8GB tf card,format to ext4.
My device is a tv box,is a android OS device.
It mount tf to /mnt/sda/sda1.
I copy /data all file to it: cp /data/* /mnt/sda/sda1 -rf
Set a flag to check the tf is mount when android start.
touch /mnt/sda/sda1/tf

write a script:/system/etc/,This file is run when system start.

while [ ! -f /mnt/sda/sda1/tf ]
sleep 1
mount -o bind /mnt/sda/sda1 /data

Android started,it show /data is 8GB.
But when I install app,reboot device ,i can't see the app,but /data/app have the corresponding apk file,/data/data have the corresponding data file,
and /data/dalvik-cache have corresponding dex file.
What problem happend?
android will search installed app when it starting.
Maybe is search before running?
which is the search app? is service installd?