I'll second the "don't use the external halyard" advice.
I have had one external halyard block fail in my 28 years of sailing, it rained down plastic and metal all over the deck. Luckily no one was on it, only the dinghy and it was still over the side.
You should always have a safety line as well. I use another halyard that I fix in place (a static line), and then use a Prusik loop to a separate carabiner. To be a true safety, the carabiner should be attached to a separate harness other than what the dynamic (hauling) line is attached to. Also, if the dynamic line were to fail, how would you get down? You've used all the halyards so rescue is going to be complicated and may take a long time.
I bring along a descender so I could get down without help if I had to. You might check out the Petzyl Grigri 2. You can clip it on to the static line and then lower yourself with it. There is a good Youtube video, search for "Mast climbing with a grigri by allen edwards".