Reformatting external drive - problem ?

I have an external drive with two partitions, from a year or two back. Today I attempted to reformat the entire drive, but I could not. On disk utility there are the two partitions, but no drive, just the partitions. I can erase the contents, but I cannot remove the partitions. Can anyone advise me of the problem, and tell me how to reformat the drive? 🙂