BACKUP YOUR DOCUMENT.
Just realized the file wasn't properly working on Google Drive. It was downloading an HTML fime. So i decided to make a ZIP file now with everything. Also put a copy in a pendrive. Just in case.
depending on how nerdy you are, you could use source control -- the kind they usually use for programming source code. it even keeps fancy diffs (the differences) between your versions.
i'll do stuff like this from my command line at the end of each session:
$: git add .
$: git commit -m "about one-third done with chapter 9"
$: git push
and i can back up and revert to any version ever.
https://bitbucket.org will give you a free private repo even.
like i said tho, it depends on your nerd comfort level.
hrm... or maybe i'll build and sell a tool that helps to automate that process for writers.