Linxers posted a guide about 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.
This space by default is around 5% of the total. And considering the amount of hard-drive or partitions available these days, this reserved memory could be huge e.g. 5% of 500GB hard disk is 25GB, which is too much for critical cases. And this memory is only needed in the root(/) partition and it doesn't make much sense to reserve blocks on /home partitions or external storage devices, because privileged processes don't usually write on home partition.

  Increase available disk space by decreasing the reserved blocks in extended filesytems ext2/3/4 using tune2fs