Potential problems with Windows Updates when Fast Startup is enabled in Windows 10

In Windows 10, Fast Startup is enabled by default, if applicable. The purpose of Fast Startup is for a faster startup following a system "shutdown" selected by the user.

However, a system shutdown in Windows 10 with Fast Startup enabled really just places the system in Hibernation mode; not a full system shutdown as one may think where a cold boot would be necessary to restart the computer.

Therefore, those Windows Updates that are "Pending Reboot" will likely not install properly if "system shutdown" has been selected to install Windows Updates (versus selecting "restart").

The solution is to select "Restart your System and Update with Windows Updates". Doing so will fully shut the system down and [cold] reboot it for you thus allowing pending Windows Updates dependent on a full system shutdown and cold reboot to install properly.

Also, please see - Disable or Enable Windows 10 Fast Startup (and Why) - Sysnative Forums

From Microsoft
» Updates may not be installed with Fast Startup in Windows 10
» Distinguishing Fast Startup from Wake-from-Hibernation - Windows drivers
» Power IRPs for the System - Windows drivers
» Assessment - Boot Performance (Fast Startup)
» Exercise 1 - Evaluate Fast Startup Using the Assessment Toolkit
» Delivering a great startup and shutdown experience
» Change which apps run automatically at startup in Windows 10
» Enable Hyper-V on Windows 10

Hibernation - Hybrid Sleep, Sleep, Hibernation Power-Down Options - Windows 10 - Sysnative Forums