Yh, sorry...
Open up Disk Utility, with the External HD Connected.
Click on the External HD's name (Click on the HD itself, not the volume, if your not sure which one is the volume just click both orange icons, and choose the one which has the partition tab to the right)
Click on the Partition Tab
From the drop down menu choose the number of partitions you wish to make, in your case you want one partition.
and then simply click on the partition button at the bottom.
CAUTION!!!: this will erase everything on your HD...