I have dual boot. Today I switched to Windows 10 and there was a red bar indication on This PC saying that my main partitions are almost full. I had no idea when I was dumping them through Ubuntu.
Please consider adding a drive space indicator with a warning for better user experience. Thanks.
Check if you have some additional folders in the root filesystem, by any chance, such as /timeshift, eating up the space in Ubuntu.
A drive space indicator is very useful thing to have, though.
I was intentionally adding files without knowing that storage space is running down as there are no indicator available.