Fixed: Windows 10 update KB4493509 fails with error code 0xc000021a

April 23, 2019 8:06 pm Published by Leave your thoughts

Note to self: Insert long and useless intro paragraph here that makes everyone lose their shit.

So I’m using a Windows 10 Professional on a desktop and a few days ago when I turned on my PC I noticed that at first it was trying to apply the updates it had downloaded the night before, but soon after failed to install them for some reason and reverted back to the original state.

This went on for a good 15 minutes too. It tried to apply the updates but then failed and the computer would reboot. At first I thought it was stuck in a reboot loop which luckily wasn’t the case. But It was still frustrating as hell. Know what I mean?

What I tried

First I tried to boot into Safe Mode and delete the SoftwareDistribution folder. After the first failed attempt I thought I should maybe try the same thing one more time just to be sure. Nope, still nothing.

How to boot Windows 10 in Safe Mode?

Well the easy way would be to open run menu using the windows key + R and then typing in msconfig and hitting the Enter key.

From the thing that pops up choose the Boot tab and tick the Safe boot option. Then restart. Your computer should now start in Safe Mode. Then just go to C:\Windows\SoftwareDistribution and delete everything inside there. Don’t worry it should be fine.

Finally just visit the boot settings again and uncheck Safe boot and you should now reboot back to your normal environment.

If this alone doesn’t work, then…

What ended up doing the trick for me was to run the Deployment Image & Servicing Management (DISM) tool to fix the corruption errors. Don’t worry if this sounds confusing! Just do the following:

  • Press the windows key on your keyboard (this time without the +R!)
  • Type in CMD and hit Enter
  • Choose Run as Administrator (for the Command Prompt tool)
  • In the Command Prompt type in sfc /scannow and hit Enter (notice the space before the /)

Now just leave it be until the process is 100% completed and then restart your system. I think I did the whole Safe Mode/delete SoftwareDistribution contents AFTER the scan.

Chances are that the updates will now download and install perfectly fine if you notice your computer tell you something about how it gracefully fixed some corruption errors it found while scanning.


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This post was written by ynef