Why is there Unallocated Space?
All hard drives, once they are formatted, start with unallocated space. In fact in normal use a large part of the hard drive will be unallocated space (if you download a LOT of movies or the like, this will not be true as you fill up your hard drive quickly).
The unallocated space only becomes allocated one data is written to it
If the hard drive starts with 500 GB of unalloacted space, when 1 GB is added there will be 499 GB of unallocated space.
The only time when there is no unallocated space at all is when the hard drive is completely full, which is unusual, and the moment a file is deleted, even a simple word document, will create some unallocated space.
Therefore the question should be:Why isn’t there unallocated space?