Mounting home folders to sd card

I just formatted the sd card to ext4. Two things:

  1. df -h reports a total space of my 128G card as 117G, used as 24K and available as 111G, why isnt the total space available?
  2. After rebooting and mounting the sd card, I have no write permissions.

Sorry for the noob questions about mounting and formatting, but I am trying to set m L5 up as a proper convergent device together with a laptop dock and for that I need a good amount of disk space on the device.