Unallocated Space

Unallocated space, sometimes called “free space”, is logical space on a hard drive that the operating system, e.g Windows, can write to. To put it another way it is the opposite of “allocated” space, which is where the operating system has already written files to.
Examples.
If the operating system writes a file to a certain space on the hard drive that part of the drive is now “allocated”, as the file is using it the space, and no other files can be written to that section. If that file is deleted then that part of the hard drive is no longer required to be “allocated” it becomes unallocated. This means that  new files can now be re-written to that location.
On a standard, working computer, files can only be written to the unallocated space.
If a newly formatted  drive is connected to a computer, virtually all of the drive space is unallocated space (a small amount of space will be taken up by files within the file system, e.g $MFT, etc). On a new drive the unallocated space is normally zeros, as files are written to the hard drive the zeros are over written with the file data