Hi there,
Felt like I needed to post here as I have been struggling with this problem also and have found a solution. This happened to me after moving a full project from one drive to another (it was contained inside a folder with all the assets.) When I opened the project on the new drive I could not burn or save as 'Save' always stuck at 70%.
After lots of fiddling this worked for me:
1. Make sure the Encore Project is closed.
2. Next to where you have saved the project file there is a folder (the project folder) with same name as your Encore Project. Open this.
3. Find the 3 files called ProjectMedia.acx, ProjectPrefs2.xml & ProjectWorkspace.xml. Delete these files.
4. Open Encore again. Encore should re-create these files (or at least the files it needs to) and you should now be able to save your project and build/burn.
Good Luck! 