If you don't have to "merge" all the data:
Connect the external drive to your PC and copy all the data from it.
Right click on "My Computer" and select Manage (Or go to Control Panel - > Admistrative Tools -> Computer Management).
Click on Disk Management and wait for the viewing pane to populate with information on all the drives on your computer.
Make sure you know which drive you want to repartition and delete all the volumes and partitions on it (be very careful here because all the data on these volumes and partitions will be lost).
Create a primary partition, making it as large as permitted (all but 8 MB, or something like that).
On the new partition, create a volume and format it (NTFS, FAT, or FAT32).
If you have to "merge" all the data, and don't want a "pay for" partitioning tool, try the open source Gnome Partition Editor on Sourceforge (http://gparted.sourceforge.net/
Hope this helps.