Please Delete the Preview Cache folder
To delete the preview cache, do this:
1. Quit Lightroom.
2. In your Operating System, locate your Lightroom catalog. Typically catalogs are in /Pictures/Lightroom on Mac and My Pictures\Lightroom on Windows.
3. Find the folder whose name ends with "Previews.lrdata" and delete it. (note, you may also have a folder whose name ends with "Smart Previews.lrdata." You shouldn't need to delete that one.)
Once the preview folder is deleted, restart Lightroom. You may see gray thumbnails in the grid for a few moments as Lightroom starts rebuilding your preview cache.
Let us know if that helps
Thanks for your response. After deleting the Previews.Iradata here’s what happens when LR 5 is opened. The same screen appears saying that LR encountered an error....... However, the new thing is that a small inset now appears at the bottom of this screen with the word “Loading”. There is a circle of dots before the word Working, but there is no movement in the dot circle. LR still shuts down when OK is clicked and the second screen saying that LR is not finished writing matadata..... does not appear any more.
Thanks in advance for any further thoughts on the problem.
It is possible that you deleted the wrong preview cache for the wrong catalog.
Please have your operating system perform a search for all folders whose name ends with Previews.LRDATA. Delete each one. (Do not delete if the name is Smart Previews.LRDATA)