Mount external sdcard obb to internal SD card

I want to move a game's obb (1GB) to an SD card. Copied the obb file to SD card, and symbolic link by: su ln -sf /storage/sdcard1/Android/obb/com.game /sdcard/Android/obb/com.game but failed with "function not implemented". I googled it, and got "some filesystem doesn't support symbolic link". So I tried to use mount by command: su mount -o bind /storage/sdcard1/Android/obb/com.game /sdcard/Android/obb/com.game but I found only root can see the files in the folder. Then I do: cd /system/bin/ chmod 4777 ./toolbox #make anyone can use mount(in toolbox) su u0_aXXX #the game's uid mount -o bind /storage/sdcard1/Android/obb/com.game /sdcard/Android/obb/com.game ls -l /sdcard/Android/obb/com.game $ -rwxrwx--- u0_aXXX sdcard_r com.game.obb But when I cat /sdcard/Android/obb/com.game/com.game.obb $ cat: /sdcard/…ame.obb permission denied I'm using Android 5, any solution? --- (I posted this question on [Stack Overflow](https://stackoverflow.com/questions/42738392/android-mount-obb-to-external-sd-card-failed) before, then I found here may be a better place to post this question.)