Recently I forced the installation of Windows 10 21H1 on my ThinkPad. To do this, I downloaded the Windows Update Assistant and ran it.
Don't try this at home.
Unlike the upgrade from the Windows Update app, this process does a FULL Windows 10 update.
But it all went well. It took a long time unlike using the Windows Update app but worked fine.
Then a week later, I was poking around in my C: drive. (You do this, don't you?)
I found several folders that I wasn't expecting:
$GetCurrent - 4.23 GBWindows10Upgrade - 3.62 GBWindows.old - 25.6 GB (that's not a typo)
Those weren't really a problem on my HD but still that's over 33GB of space.
Surely Windows 10 would clean those up. Some of them are supposed to be cleaned up
30 10 days after the upgrade. That period had not lapsed.
Windows 10 has a Storage Sense feature that has an option to "Delete previous versions of Windows".
I ran that and it reported that it cleaned up 17.4 GB by deleting Windows.old. That's a nice start.
Now you ask why did it only clean up 17.4 GB if Windows File Explorer said that Windows.old was 25.6 GB? Read this until your head hurts.
Windows 10 Forums said that uninstalling the Windows Update Assistant will delete the Windows10Upgrade folder. I uninstalled the Windows Update Assistant and the Windows10Upgrade folder was gone.
How-To Geek said that the $GetCurrent folder can be deleted but should be deleted automatically. After 10 days, its size was only 181 KB.