Vagrant::Downloaders::File Error: path to a file doesn't exist
So, another issue with vagrant adding boxes on Windows under cygwin.
Apparently box add isn't working when I try and source a local file, but downloading and adding from HTTP works fine.
On further investigation, it appears that it fails to recognize the paths when given in unix format (i.e. starting with a slash or tilde). However, when I specify the filename starting at the C:/ drive, vagrant imports the box fine. Not sure how to fix this in a cross-platform manner, as cygwin does expand the tilde, but leaves it relative to it's root, without C:/cygwin in front.