I just helped someone unbork their Ubuntu system over the phone. It was reporting “disk full”, without being more specific than that. I got him to open up a terminal window, and use the “df” and “du” commands to narrow down the directory that was filling up the disk. Sometimes /tmp gets filled up with stuff, but in this case it was /var/lib/apt/lists/partial/. I have had an instance of high disk usage on that directory myself, a while back. I think it’s where apt keeps partially-downloaded repo index files, though I’m not sure why—is it to retry downloading the rest? But when the problem hits, it just seems to fill up with failed downloads. So I carefully led him up to executing the command “sudo rm /var/lib/apt/lists/partial/*”, and that cleared the problem. The main thing I was careful with was spelling out the commands phonetically, and getting him to read them back to me. But imagine having to do this with a GUI, over the phone. I don’t think I would even try...