You cannot mount it because you are runnig the system, you cannot overlay a mount when you have LOTS of open files there currently How to check i now; Create another directory where you can do the mount, or if for example your /mnt/ is unmounted, do: Code: mount -t ext4 /dev/mmcblk0p3 /mnt/ Then just check what you have on the /mnt/ When you can check that you actually HAVE the stuff that's supposed to be there, you can boot your device in maintanance mode, copy the recent stuff that you have in your rootFS /home/ to mmcblk0p3, delete all stuff under your rootFS /home/ to save rootfs space, and finally correct the thing why it does not mount it in boot phase. I suspect you have played with your /sbin/preinit and changed the mount order...
mount -t ext4 /dev/mmcblk0p3 /mnt/