external ethernet adapter is already bound to the microsoft virtual switch protocol
After a quick google search I came across a MS support article which described this error to a T.
You need to download the NVSPBind tool seperately from the MSDN website here. After opening the command prompt (in elavated administrator) I ran this command:
nvspbind /u “Friendly NIC Name”
When this was finished I was able to configure the virtual switch to connect to my external NIC.