So my Windows 10 installation was throwing this update error 0x80070643 since a last few days:

Went through many solutions such as resetting the windows update service, removing downloaded files, etc. but nothing helped to solve the problem. The thing is, Windows 10 seems to hide the actual error codes behind some generic error codes so you actually do not know what is causing the error 😐
Finally I found this helpful blog post A broken Windows Defender update which had some steps how to get detailed log of why it’s failing.

I ran Get-WindowsUpdateLog in PowerShell and went through the update log file where I found the error code why it was actually failing – 0x80092003. Let’s see what this error code means – all the Windows error codes seems have been documented on COM Error Codes (Security and Setup).
So the error code 0x80092003 means CRYPT_E_FILE_ERROR which is basically “An error occurred while reading or writing to a file.”. It doesn’t specify what is the error though!

I made a wild guess, that probably it is not able to write to a temporary file or something. On my system temporary folders were set to be on RAMDISK using IMDisk Toolkit. So I went to the environment variables page and changed the system TMP and TEMP variables (only) to point to the original %windir%\Temp folder and rebooted. Then I tried to run the update again, and it worked! My personal temporary folder continues to be on RAMDISK.

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