Hmm - if I’m not mistaken, is this trying to use or access my “D:” drive?
If so, that is not storage, but an optical (dvd) drive with no media in it.
Could that be the cause?
Well would you believe it - my suspicions have been confirmed. 
In Windows Computer Management I remapped the drive letter of my optical drive from D: to Z: then tried to uninstall MSYS64. The program wouldn’t even begin to uninstall, so I just deleted the folder from the C: drive, and removed the entry from programs and features in the control panel.
I then reinstalled MSYS64 and ran through the “Installing On Windows” guide, exactly as printed and just as before. Bingo - not one error and Siril is now running as it should.
So, this begs the question - why on earth would MSYS64 attempt to store temp files on my only, non-writeable drive? I have 4 other SSDs and HDDs in my PC, yet for some reason it opts to install on an empty DVD drive instead. 
Thanks for the assistance.
Mark
Complete mystery as to why this would happen. Seems PYTHONHOME was not set during your first install with msys2, nothing you should normally have to worry about… Good news changing the drive letter fixed it. Enjoy the dev version 
C.
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