I must remember in the future to avoid certain functions on the 8.5a. One is the FIND button which will cause the tree to abort. The same now happens with using the GedCom function for export. These and certain other functions seem not able to handle the large size of my main tree (1 Gb), but work okay on other small trees.
Going back to a tree several days old, and stored on flash drive, I was not able to get 25% of the way through "validating," and couldn't reach "indexing." Not even with dozens of attempts. By finding an even older saved copy from 2 weeks ago, and with great patience, I was able to restore it to my XP.
By great patience, I mean that the trials to bring it up went past "validation" and the usual tiny bit into "indexing" before freezing. From much experience, I knew this to be a ray of hope -- that eventually a trial would be successful. And so it was at sundown yesterday that the monster-baby, that I have nurtured for so long, came up.
The downside is trepidation in using what is obviously so fragile a file, and the loss of about 50 hours of hard work -- amounting to the loss of about 1000 people -- the ones most recently posted in the tree.
The upside is that despair has given way, yet again, to hope and happiness. Why so big a project? I noticed years ago that nearly everyone in my home county has some kind of kinship, so I have (in my retirement) been working on a county tree. Since I am the volunteer archivist at our local genealogy center, the tree is a priceless tool in helping with the searches of visitors.