|Connect the drive to your PC and make sure the enclosure is connected to mains electric socket.|
Go to Control Panel > Admin Tools > Computer Management > Disk Management
In the right-hand pane, at the bottom, find the appropriate ribbon for that drive.
Right-click each of it's partitions in turn and choose "Delete Partition".
When they've gone you will be left with one large "unallocated" space.
Right-click that and choose "create partition", accept the default size.
When partition has been created, right-click it and choose "Format".
When that's finished you should have a single partition using the drive's total capacity and ready to accept data.