Windows upgrades used to be something you only had to worry about every few years. But in the Windows 10 era, each twice-yearly feature update is essentially a full upgrade.
When an upgrade or feature update fails, Windows Setup typically doesn't provide any obvious indication of the underlying problem. But it does keep detailed records of every activity as it works, and if you know how to read setup log files, you can often pinpoint the issue.
These log files are typically saved in a compressed folder called Panther. (The exact location of the Panther folder when an upgrade fails depends on what stage Setup was in when the failure occurred.) If Setup fails and rolls back to the previous Windows version, the log files are stored in a folder called $windows.~bt\Sources\Rollback.
Reading raw log files requires expert skills and specialized tools. A much simpler option is to use a new diagnostic utility, SetupDiag.exe, which is designed specifically to read log files and generate a report identifying the most likely problems.
You'll find full instructions for using SetupDiag at this page, which also includes a download link.
Copy SetupDiag.exe to its own folder and double-click to identify setup problems on the current PC. The program is smart enough to look in locations where log files are commonly saved, so you don't need to specify any command-line switches for the tool.
If you're diagnosing a problem on a different PC, copy the folders containing the log files to a local folder. Then run SetupDiag using the /Mode:Offline switch and the /LogsPath:<folder> parameter, replacing <folder> with the name of the location where you copied the log files.
The output for SetupDiag is a plain text file that is saved in the same folder as the program file. You can open the results file in any text editor, including Notepad, to examine its output and figure out what to do next.
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