You should really add how to partition without losing data.
1) Get Partition Magic 9 (pirate if need be
)
2) Install, run and then select create new partition
3) Select your external drive with some free space on it.
4) Select the size of the new partition depending on the free space, select a drive letter, volume label etc.
5) Select a filesystem to format with (Don't worry you can change it to WBFS later using one of the GUIs)
6) Wait for a while till it creates the new partition.
7)Voila! You now have a new partition to boot your Wii games from without losing existing data! Format it to WBFS and add your games.
Hope this helps.