Recently I was looking at my file system space and noticed things didn’t quite add up. The space available and free space were not quite right. After doing some digging I discovered that ext4 reserves a small amount of space on the drive for the root files on the system.
In my case root is on its own seperate drive so this feature is pretty useless to me. Found an askubuntu post (https://askubuntu.com/questions/249387/df-h-used-space-avail-free-space-is-less-than-the-total-size-of-home) about this issue and it is quite simply a case of:
To see information about reserved blocks
sudo tune2fs -l /dev/sda8
To set the reserved blocks to 0:
sudo tune2fs -m 0 /dev/sda8