Unhappy [Q] Cannot boot by /system on SDCard

I had move the /system partition on sd card, and modified the init file to mount sdcard partition when booting. But it failed and the phone stayed at logo screen.

I create a FAT32 partition and ext4 partitions on sd card with MiniTool partition Wizard, and mount them under /mnt directory.

Then, copy /system partition to sdcard, by cp -a /system /mnt/system.

Finally, I extract the boot partition, modified the init file in ramdisk directory, repack and flash back to internal memory.
The mount info located in init.smdk4x12.rc as mount_all fstab.smdk4x12. So I modify the mount point of /system as sdcard partition.

The phone cannot boot, and stay at logo screen.

However, I use the same method to /cache and /data partitions, it works very well

So, is there anything wrong with this method for /system partition?
Thanks for your help!!!