So, after reading another post on the Failed to save issue, I think I have found the culprit that is causing this error…for me. If you use the Print option to create PDF files of your model, this can cause the Failed to save error. I was able to create a very simple test case that documents the process that can cause the Failed to save error and the subsequent hanging of the SU process if you try to download your model after getting this error.
If you read my latest post in the referenced thread, you can download a copy of the bug report I submitted regarding this problem to see how simple it is to create this condition.