USB flash drives (also known as "pen drives") are divided into partitions. Each partition is independently formatted before storing data by creating a filesystem. Depending on the configuration of the drive and of the operating system, a computer accessing the flash drive may be able to perform only reads from one of the partitions. You can remove a write-protected partition to reclaim the storage it occupies.
Log in to the computer as Administrator.
Check for a write-protect switch on the flash drive. They typically look like an externally-accessible slider on the body of the drive. If you find it, slide it to the unlocked position.
Connect the drive to the computer by plugging it into an open USB port.
Click "Start," then "Control Panel," "System and Maintenance" and "Administrative Tools." Double-click "Computer Management." Navigate to "Storage" and then to "Disk Management." Select the flash drive by clicking on its entry on the lower half of the Disk Management window.
Locate the partition you want to remove on the map of the space on the drive (depicted in the upper half of the Disk Management window.) Right-click on that partition, then select "Delete Partition." Click "Yes" to confirm your choice. At that point, the partition is completely erased from the flash drive.