Unallocated space is simply the space in a computer's hard drive that is not being used to store information. The actual amount of unallocated space is only limited by the size of the disk. Unallocated space is not normally accessible.

Unallocated space is often the result of a file being deleted. When a file is erased, and the space becomes unallocated, the space is not actually erased. The space just becomes available for storing new information. Until these unallocated spaces are reused, the old data usually remains. Forensic techniques can access this information.