Likely is a windows updates.

Try running Disk clean up

Click on start, then type "disk"

Click "Disk Clean Up"

tick System files

You should be able to clean up most unneeded files with Microsoft's own tools

That's just the dust buildup on the HD discs.You could just open it up and clean them with a rag, but I find it quicker and better to just machine wash and tumble dry the whole unit.

This.

It's probably GoG Galaxy's logging, there have been a couple of threads here where the log was 10-15GB.

I don't use it, but apparently if you disable the ingame overlay it resolves it.

I know. There's only a minority of us who come from the days where computers only had 512 mb of RAM and 40 gb hard drive and even less from the Amiga and Lisa days.

Actually this is a common problem ever since... well Windows. MS has just found more ways to keep this game interesting over the years (like polluting GB of space with Windows 10 upgrade installers on machines whose users clearly said "Nope, don't want it!".

Also a lot of applications just eat disk space by logging events and errors (IIS comes to mind...) which you can easily detect with tools like TreeSize. As some people state, Galaxy has been a culprit as well in this regard. It didn't happen to me, but it's worth a check - after all it's still "beta" (Which Windows 10 is almost officially perpetually...).

And yes, I belong to the people who actually notice 20 GB. For once my gaming HDD has 400 GB (which used to be a lot when I installed it around 2010) and has only 11 GB free which isn't enough for a single AAA title nowadays. And also downloading 20 GB would probably block my internet connection for about a day...