It is very common to encounter this, especially on larger disks. In most cases there is really nothing wrong, except that a substantial portion of the storage area of the disk is wasted. In addition, various software packages can consume large amounts of hidden storage. File system corruption can legitimately lead to lost storage space. Since the FAT file system allocates space in chunks called clusters, each file can only use a whole number of clusters.
Slack is discussed in much more detail here .
Scan the hard disk for file system corruption . You should check for file system problems regularly as part of your system care routine. Fixing the lost clusters will free up this space. Usually only a small number of clusters are lost at a given time. If you see lost clusters regularly, troubleshoot that problem here .