How to properly wipe a WDE external drive?

Ladies and gents, I dun goofed big time.

I had installed PGP Desktop on a Windows 8 to go which is installed on an external USB disk. I had used WDE to encrypt about 16GB of the one TB total space before I came to a horrible realization in this order:

1. The key pair used is only located on the external USB disk.
2. How is WDE occurring when the key pair is on the disk being encrypted?
3. I am unable to boot from the external disk any longer.

Being that I cannot decrypt the disk, is there a way in which I could wipe it? I have tried Diskpart ---> Select Disk ---> 'clean all' for a few hours but the disk is still considered encrypted.

Required info:

PGP Desktop 10.2.1 Build 4940 SDK 4.2.1

Windows 7 Ultimate 64bit