A family of Microsoft word processing software products for creating web, email, and print documents.
Are you using Google Drive? If so, try turning it off or setting GDrive Sync off. Likewise, if you are using any Internet drive service, like DropBox or Syncplicity, try turning them off. In all likelihood, the problem resides in certain process still locking the large file when Word attempts to save it. Thus, doing a Save As ensures no one else is using it and the operation succeeds.