unallocated disk space (n)~ Any unpartitioned and unformatted space on a hard disk. unallocated space (n)~ Available disk space that is not allocated to any volume. The type of volume that you can create on unallocated space depends on the disk type. On basic disks you can use unallocated space to create primary or extended partitions. On dynamic disks you can use unallocated space to create dynamic volumes.