I have a Jupyter notebook called "Visual Magnitude.ipynb" on my Windows box. I've used this notebook for several years. It's under CM control (perforce is our CM tool). Normally I don't have it checked out, so it's marked as read-only on the file system. Sometimes I open it up (knowing it's read-only), add a few cells, look at some results, and close it out, knowing the new cells won't be saved. This is ok. 
But lately I've run into a situation where I forgot to check it out of perforce first, then added/modified some cells. When I went to save it Jupyter complained it was read-only. So I checked it out (thus removing the read-only status on the file system). Jupyter still doesn't recognize this. So I quit Jupyter all together and restart it with the notebook (which is now writable). But Jupyter refuses to recognize this and still treats it as if it were locked. Almost as if it's caching the file status in some location.
I've rebooted and still have the same problem. What am I missing to convince Jupyter that this notebook is now writable?
I'm using Jupyter 4.4.0.
It turns out that there was a hidden file .~Visual Magnitude.ipynb in the same folder that was still marked read only. Once I deleted that everything was fine.
Jupyter notebook won't let me save itself