I discovered the C drive on my main Windows 10 system was almost full. I was not expecting this, since I use Linux most of the time. A lot of the trouble was a file in the root called hiberfil.sys, taking up 13GB. This is used if you want to hibernate your Windows system, and needs at least 40% of your RAM. I never use hibernation, I simply disabled it in an administrator command window with

powercfg -h off

and the file disappeared, as described in How to Disable Hibernation on Windows 10. I also moved a few large iso files to other locations and now have a comfortable amount of free space on the drive.