With thanks to Martin Hargreaves for the detective work.
When you build a DVD, Encore displays one of the following error messages and the burn fails:
Operating System Error: number = 0X00000020. The process cannot access the file because it is being used by another process. This occurs because you have rendered your motion menu then edited it by removing the motion background or by changing its layout or text.
Delete the rendered motion menu files from the project folder
In Windows Explorer, Open the project folder and navigate to the Sources/Menus folder. This folder contains Encore's working copies of all the menu files and motion menu backgrounds. The name of the menus are not immediately obvious so you will need to identify which menu is the one from which you removed the motion menu background. You can double-click the .psd files to open them in Photoshop or import them into any Encore DVD project. Once you have identified the problem menu, delete the .mpg and .xmpses files in the Sources/Menus folder that share the same filename as the .psd file.
If your menu is named Menu_Bpgc_Asf_539856144.psd, delete the Menu_Bpgc_Asf_539856144.mpg and Menu_Bpgc_Asf_539856144.xmpses files.
Note: Do not delete .mpg or .xmpses files for menus that still contain motion menu backgrounds. Doing so may cause additional errors or Encore DVD to freeze or fail during the build process.
Solution 3: Troubleshoot menus to determine which one is is causing the error
1. Save the project with a new name (this is for troubleshooting purposes only).
2. Save each motion menu as a template by clicking its icon in the Project panel and choosing Menu > Save as Template.
3. Delete one menu and try rebuilding the project.
4. If the error persists, continue deleting menus until you determine the one that is causing the error.
5. When you have determined which is the problem menu, reopen the original project.
6. Delete the problem menu by clicking its icon in the Project panel and pressing delete.
Note: If the problem menu is a motion menu, delete the source video for the motion background as well.
7. Reimport the menu using the templates you saved in step 2. If the problem menu is a motion menu, open the .em file instead of the .psd file to add the menu and the motion background.
Note: You can also add the saved .psd and .em files to the Library by clicking the Library flyout menu and choosing Add Item.
8. Relink the menu's buttons.
9. Rebuild the project.
These errors occur because Encore is trying to overwrite the temp file for the motion menu and it was locked when the motion menu was originally rendered.