You didn't mention how big your hard disk is, so I'm not sure if this is the solution for you. Anyway, by default on ext4 filesystems 5% of the disk space is reserved for the root users and not usable by you. That is done so that in case of you filling up your hard disk, and having problems booting or logging in because of that, you could still boot and log in as the root user as that has 5% free disk space reserved.
If you want, you can remove that reservation. It was about another question, but see this post for how to do that: viewtopic.php?f=6&t=114355#p636652