So far I've installed the Windows 10 Fall Creators Update on 7 machines. Those machines include 2 desktops (one running the Windows Insider Preview slow ring and one stock Windows 10) and 5 laptops (one running the Windows Insider Preview fast ring and the others all stock). The machines are from a variety of sources: one desktop was purchased and one was built, and the laptops are a mix of HP, Toshiba, Microsoft, ASUS and Lenovo.
The upgrade experience to the FCU has been mostly smooth so far aside from two issues, one minor and one catastrophic. The minor issue is around ASP.NET app pools in local IIS, which I detailed in a previous post. The catastrophic issue occured on my main desktop machine. The update appeared to run smoothly and complete successfully. However, when it rebooted to complete the update, I was met with a black screen and the message:
Reboot and select proper Boot Deviceor Insert Boot Media in selected Boot device
I tried all manner of fixes, including every combination of boot device order, trying to run a recovery from a Windows 10 install DVD, trying to use a partition tool to check partition health, and so forth. There was nothing wrong with the drive, the partitions were all still there and intact, I could access the data just fine using a Linux recovery disk, and so forth. It just refused to boot.
This is a stock PC purchased from Micro Center (their brand) and came with Windows 10 Pro installed. The only upgrade I've made to it was to swap the preinstalled main drive, a 250 GB SSD, with a larger 500 GB SSD. And that is my only guess as to what may have gone wrong.
I had used Acronis to copy the image from the 256GB drive straight across to the 500GB drive just expanding the C: drive to fill the empty space. However, I left the 256 GB drive installed and unaltered after it was done. All I did was change the boot order to put the 500GB drive first and changed the drive letter on the other drive's Windows partition. Therefore the machine had two drives that thought they were the Windows boot drive.
This is only my theory as to what happened, but I'm guessing that somehow the update couldn't tell the two "boot" drives apart and somehow made some fatal alteration to the boot record on the 500GB drive, or maybe both of the drives, leaving the boot process unable to run and properly start Windows.
Regardless of whatever the culprit really was, after about 8 hours of effort, I gave up trying to recover the drive and wiped it and re-installed Windows 10 from scratch. I do have file backups (both local and in the cloud), but not a drive image backup, so I'm now in the process of re-installing my applications on the machine and restoring my files to their proper places.
It's not a big deal and, as I said, the other machines all updated just fine. As someone who has tinkered with PCs since my dad brought home a ZX-81 lo those many eons ago, and as someone who is a professional and private app developer, I take things like this in stride. It happens. And if my theory is even close to correct, it's not something that's likely to affect "normal" people, just the geeks who like to tinker and upgrade their hardware on their own. So if that popup tells you that your machine is ready for the Fall Creators Update, don't hesitate. Click the button and get your upgrade on.