This situation is
inexcusable on Microsoft's part. I run 5 computers in my home, 1 of which has been in the Windows 10 Insider program since its inception. This problem never showed up in all the Insider Preview releases. On October 3, I updated the 4 other computers
to 1809 with no problem and no lost files that I have been able to detect.
I have a question for those who say that they have lost many years of work. What backup system do you have in place? This situation is
no different than it would have been had your hard drive failed. How would you have recovered your files in that situation?
Hard drives can fail at any time, whether they are 10 days old or 10 years old. Multiple backups of data files are essential at all times.
Frankly, Microsoft needs to abandon the
twice a year **"feature update"**approach. One time per year is enough. Even when things go smoothly, twice yearly is far too frequent for home users and for businesses. Let's all use this as a tough learning opportunity, and that goes 10-fold for Microsoft!