How to write an email to a potential PhD advisor that I have worked with in the past I am just finishing up an MSc degree in Canada and am looking to begin sending inquiry emails to potential supervisors regarding available PhD positions (also in Canada). However, I am unsure how to approach one specific scenario: 

I am looking to email a potential supervisor who I have worked with in the past although not very closely as he has never been a direct supervisor and is at a different university. However, he still knows most of the pertinent information that I would typically include in an inquiry email (e.g. current school/program, research topic, GPA, etc.). He also knows that I am aware of his current research and his recent publications as my research is closely related and we have completed fieldwork together that is related to both of our topics. 

I am unsure of how formal to be or what to include in my email given that we do have somewhat of a pre-existing relationship. Typically I would try to speak in person with anyone that I already know but given that he is at a different university that wouldn't be possible without advance planning. 

Any guidance on what to include in an email asking about potential PhD openings and hopefully arranging a meeting to discuss any opportunities in person would be much appreciated!
In my opinion, what works best is a personal email (you probably know best how personal is appropriate with that concrete person), with a formal application attached. He might want to discuss your application with some colleagues who do not know you, or with HR. Thus, having the pertinent information will be an advantage. 

In the personal email you might want to mention that you enjoyed the common work you did previously, and that you would like to continue / build up on that, now that you finished the MSc.

Say very directly, that you would prefer his research group over others. And then add something like: in case he has any open PhD positions in the near future, your formal application and CV are attached.