Pressing the Update Media Library button results in the following:
- The media library is not updated
- The Progress bar is only about 50% of the way across
- A message appears telling me the library update was successful, but it doesn't return to the library like it normally does when complete
This means that you have uploaded a corrupt file. Login via FTP as you did to upload your media and look for any recently uploaded files that are not the same file size as they are on your computer. In particular, look for the media of size '0'. This indicates a corrupt upload and should be removed to fix the problem.