I use GitHub for Mac version 210 on Mac OS X 10.10. Hitting the "sync" button deleted my uncommitted changes. Is that supposed to happen from time to time? I've never had this issue till then, though I've mostly used Github for Windows.
I thought that if I had some uncommitted changes that conflict with whatever there is in the GitHub repository, hitting the "sync" button would fail and yield the following error message:
The Github Desktop creates an stash with your changes.
You can list the stashes with:
git stash list
And apply the one that Github Desktop created by doing:
git stash apply stash@{0}
For more information about git stash
see this.
Command line is the way to go. I faced the same situation. When I did a stash list in the Terminal it showed me "GitHub: Stashing to pull in remote changes". And I was able to retrieve my local uncommitted changes back again. :)
It did happen before (even for GitHub for Windows).
For added changes to the index, a git fsck --full --unreachable --no-reflog
could help (as I mentioned here)