The most notable issue, and likely most critical, involved with the update is that it is causing Blue Screen of Death (BSOD) errors for some. Users on Microsoft's Answers forum report that their machines are encountering a BSOD after installing the update, preventing the systems from booting (via Neowin). The issue appears to be affecting systems from different manufacturers, even stretching to custom built desktops. Says one report:
I deploy patches via SCCM on a regular basis to nearly 5500 clients worldwide. My first group of Alpha systems which contains all of my own workstations receive the first round of patches as a test. ALL of the systems running 1703 blue screen upon reboot AFTER applying this patch (KB4041676).
The message that comes up is INACCESSIBLE BOOT DEVICE. After that, it reboots automatically, attempts to boot again, and then displays the "Automatic Repair" screen. None of these systems make it past this message.
It's not clear how widespread the BSOD problem is, but it's a substantial issue if your machine happened to update overnight and will no longer boot.
In addition to the BSOD problem, Reddit users are complaining of other issues. Some say the update is maxing out their disk usage, while others are reporting the update is getting stuck while installing.
We'll update this post with more information if and when Microsoft addresses the issues. In the meantime, if you haven't yet installed build 15063.674 (KB4041676) you may want to either hold off for now or proceed with caution.
Updated October 11, 2017: The BSOD issue appears to only affect machines behind a WSUS in enterprise environments. Most users simply downloading via Windows update should remain unaffected. Microsoft MVP Susan Bradley explains on the Answers forum:
Assuming my theory is right and you guys caught the delta patch problem -- Bottom line Microsoft accidentally pushed the Delta updates to the WSUS channel. If you happen to have caught both, this will bsod a machine.
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