Am I just going about it the wrong way or should I look for a third party software? I am on Yosemite 10.10.3
You need to explain a bit more. Sorry, but it isi not clear, what you already have done, and what the results are.
Have you already started to migrate your iPhoto Library to Photos? What happens?
My library is on an external drive I have researched other people who have abhorted this because it takes ages & at times seems to stop etc
An external drive is no problem, if the drive is correctly formatted and directly connected. Is the filesystem on your external drive MacOS Extended (Journaled)? Is the drive directly connected or is it a network volume?
When you first launch Photos it will automatically the last used iPhoto or Aperture library to Photos, if it can access it.
BTW, it is better not to mark a question as solved, before you even have the first answer. Then nobody may bother to look at the question, thinking it has already been answered.