There is an update that is broken like this that I think Grif has written about a few times. I don't recall if there was a fix as we wait for Microsoft to correct.
The times I ran into this I lucked out it was a family member's laptop and I got it before they took it to the shop or Microsoft. On startup I caught it trying to do a check disk and fail. Your everyday user may not have noticed this or recognized the messages. SO IN THIS CASE all I did was run chkdsk with the full /F /R /X switches on all drive letters and after that completed the updates finished.
Mind you that today folk don't expect to be IT specialists so at some point Microsoft needs to cover that issue. Here's a link about chkdsk and /F /R /X. https://www.tekrevue.com/tip/fix-hard-drives-chkdsk-windows-10/
Now it could be some other issue like insufficient space or some incompatible app but I'll share what I've run into.
My Dell desktop has been trying to install the cumulative update for Windows 10 1709 for x64 based systems since January. It says it needs to restart to finish installing. I have even chatted with Microsoft and gave them control of my computer. The tech said it was installed but the computer didn't recognize it. For about 2 weeks it didn't try to install but now the message is back. What to do???

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