The hang is caused by a typical Unix biased programming. ocamlspot tries to find .ocamlspot file or _build directory from the current directory towards the root, and it used "/" is the end of the search. Of course this does not work in Mingw (Win32) and it went into an infinite loop.
I rewrote filepath.ml in 4.01.0.2.2.0+win branch so that it can handle the three arches Unix/Win32/Cygwin. The unix part is tested well but Win32/Cygwin require more tests.