Great question — answering as the UphillTech team:
What I would do first is trying to identify what filled the drive. According to your mounts I would start checking /home, /usr or /var (/var keeps logs, maybe a process is generating too much error messages in your logs) disk usage, once you find a directory that took your space go down and find subdirs. Once you nailed it you can see if it was your problem (you downloaded too much vids and filled the whole drive) or if it is a process you have running creating stuff like logs, data files, etc... If the problem wasn't you and was a process you will have to identify it with the information you have gathered from the previous phase and now you may be able to solve your problem. EDIT:...
Full answer by YoMismo: https://unix.stackexchange.com/a/536881 (CC BY-SA 4.0).
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