Can't mount /data, /cache, and /sdcard
There are a couple ways to do this:


Using adb

create a folder named external_sd in /sdcard/  with your file manager

Use  this code in commamd prompt (adb) (to make this folder to mount sd card in /sdcard/):


su

    mount -o bind /mnt/emmc /sdcard/external_sd



Alternatively via terminal (need busybox installed and root):

busybox mount --bind /mnt/emmc /mnt/sdcard/external_sd (as su)


Credits

https://forum.xda-developers.com/showthread.php?t=1724278