ext2, ext3 and ext4 filesystems by default reserves 5% of filesystem blocks for use by privileged processes. On modern drives with a lot of space, that can result in much lost space.

Fortunately, that behavior can be easily changed with command tune2fs.

For example, partition /dev/sdb5 have 384MB and default reserved blocks:

nevermind1:/# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sdb5             384G  195M  364G   1% /data2

nevermind1:/# tune2fs -l /dev/sdb5 | grep -i "block count"
Block count:              102077002
Reserved block count:     5103850

We can change reserved blocks percentage with command (executed by root):

nevermind1:/# tune2fs -m 1 /dev/sdb5
tune2fs 1.41.3 (12-Oct-2008)
Setting reserved blocks percentage to 1% (581231 blocks)

If we checks free space again:

nevermind1:/# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sdb5             384G  195M  380G   1% /data2

nevermind1:/# tune2fs -l /dev/sdb5 | grep -i "block count"
Block count:              102077002
Reserved block count:     1020770

As you can see, 16GB is freed.

/dev/sdb5