Instructions
Open "WBFS Manager" with your external hard drive plugged in. Input the drive letter of your WBFS partition.
Back up all your games to your computer by selecting them all and dragging and dropping them from the right to the left pane. Close "WBFS Manager."
Open "Disk Management" in Windows, or "Disk Utility" in OSX. Alternatively, you may use third-party disk management software.
Right-click the WBFS partition, and choose the option to delete it.
Click on the remaining FAT32 partition. If you want to end up with a larger WBFS partition, decrease the size of the FAT32 volume. If you want a smaller WBFS partition, increase the size of the FAT32 volume.
Add a new partition in your disk-management utility. When prompted to format it, click "No." Remember the drive letter assigned to your new partition.
Open "WBFS Manager." Input the drive letter of your new partition and click "Yes" to format it.
Drag and drop all your backed up games to the new WBFS partition.