By default, ext4 reserves space that only root can use. Makes sense for system partitions.
But for e.g. a storage drive I'd rather have that 5% to use for files.

See [this askubuntu](https://askubuntu.com/questions/249387/df-h-used-space-avail-free-space-is-less-than-the-total-size-of-home)

```
sudo tune2fs -l /dev/...     # list details
sudo tune2fs -m 0 /dev/...   # set reserved to 0
```
When making a new ext4
```
sudo mkfs.ext4 ... -m 0 /dev/...
```