There's an unknown space that is used on my hard drive. I've already use DaisyDisk and OmniDiskSweeper but both haven't detected anything.
If you don't have any data on the drive, then format it. The data is probably a malfunctioning HFS+ journal.
If you do, it is either a HFS+ journal, or some cached data. It could also be the .Trashes folder. If it is, try clearing the trash with the drive plugged in, or just type
sudo rm -rf "/Volumes/DriveName/.Trashes".