I just ran into this again and I had to spend more time and effort than I or someone else probably wants to find a good answer.

The problem is that too many people answering these questions only suggest `update-grub` (see [1][1], [2][2]).

The best answer I could find so far is [this one][3] on https://askubuntu.com/q/197868/40581. This should be a extended to **signpost answer** covering typical cases why `os-prober` is unable to find any installed version of Windows (or any other operating system).

Braiam's approach was to hope that the system will heal itself. From my point of view this hasn't shown any progress since then, it has failed and needs our support to find a good answer to this question.


  [1]: https://askubuntu.com/a/840928/40581
  [2]: https://askubuntu.com/a/848878/40581
  [3]: https://askubuntu.com/a/198875/40581