[Milhouse]

The second partition (/dev/mmcblk0p2) must be formatted with ext4 (or ext2), and when OpenELEC boots it will populate the partition with any required files and directories so yes, a completely empty second partition is fine, and you don't need to copy any files to it before the first boot.

As the partition can't be mounted, are you sure your second partition has been formatted with an ext4/ext2 filesystem? I'm not familiar with MiniTool, but just creating the partition isn't enough - once the partition is created you'll also need to create a valid filesystem in the new partition.