Do you mean there is "unallocated space" on the drive?
If yes, it is not a partition at all. You have to create a partition there using Disk Management:
Control Panel > Admin Tools > Computer Management > Disk Management
Right-click the unallocated space and choose "Create new simple volume".
When the new partition has been created (it only takes a couple of seconds), right-click it again
and choose "Format".