Reserved blocksize

Some percent of disk space on an ext file system is reserved for root (typically 5%). This can be reduced to 1% with

tune2fs -m 1 /dev/sdX #  (where X = drive/partition, like /dev/sda1)

Alternatively, the reserved block size can be specified when formatting the disk:

mkfs.ext4 -m 1 /dev/<device name>

Check the current percentage with

dumpe2fs -h /dev/sdX

You get a raw block count and reserved block count, from which you can calculate the percentage.