Hi Billabong,

Sorry, I thought u did not see those shapes, and by no means I wanted to criticize your work in a bad way I just wanted to give u a general advice that's it. and I know with the dedication and interest you are showing you will soon become good at it, keep up the good work (and of course the more organized you try to be the better (general advice))

and I think I know how Michael did fix your drone. but it is a bit complicated.

1-try to copy edges on those surfaces even the openings .
2-join the edges.
3-delete all the problematic surfaces (I thing there are four of them)
4-make a new sphere and trim it using the edges you preserved during steps 1&2.
5-join the surfaces together.(it should work)

Note: because there are tiny gaps between joined edges of your model you might need to tweak the edges over there to get a satisfying result (at least when I tried to repair it they made problem for me). and if MOI says "it is a joined surface" but doesn't show any naked edges. I suggest you to save your work close MOI and reopen it then you should see MOI says your object is "solid"

Maybe Michael can show you his processes (which I think might be more efficient) in series of pictures or video but for me it is not possible at the moment.

-Psygorn