Increase available disk space by decreasing the reserved blocks in extended filesytems ext2/3/4 using tune2fs
Have you ever noticed that after formatting a partition or external storage device - hard disk etc. into ext2/3/4 decreases the total available disk space by some amount? That's because the file-system reserves some part of the space to privileged processes. This is done to make sure that in case of file-system fill up, when user processes may not be allowed to write to disk, privilege system processes(or root user) may still be able to function properly(write on disk) e.g. syslogd etc.