Default [Q] Stuck in Boot Loop Because of vold.fstab

Anyone, please I need help. After I have rooted my friends' android tablet he asked me to swap the internal storage and the external. after editing the vold.fstab, I've restarted the device. And it just got stuck at the boot logo. I am new here at xda and i don't know how to do a nandroid backup before editing the vold.fstab file. i've tried methods like pulling the vold.fstab using adb, but no luck because permission denied. Please Help...