Reformating an External Hard Drive

Hi, I hope you can help me I was also doing the same thing reinstalling or actually reformating my hard drive when i noticed that my external hard drive was still connected and already been deleted. So I disconnected my hard drive and reinstalled Windows Vista then found and downloaded some software to help recover the files which worked but now I have no format on my external hard drive and can't see it unless i start to install Windows XP but then I can't format the external HD so hopefully someone can come up with a solution?

Thanks