So there I was quietly working away in emacs when suddenly the tab-bar and the mode-line started flashing purple! Disaster had struck, but what disaster? In the mini-buffer it was saying that there was no free space left on the drive, and so it was unable to save a file. No free space? No way, there should be about 140 gigs of hard drive space? Where had it gone?
'Conky' confirmed that the drive was full, and that it had not all gone into the trash! I had been downloading a whole slew of mails from gmail into GNUs and when I realised that it was going to be 1000+ emails, I'd panicked and killed the process. So somewhere all those emails had gone, but where?
I started 'ncdu' which is "Ncdu is a ncurses-based du viewer. It provides a fast and easy-to-use interface through famous du utility. It allows one to browse through the directories and show percentages of disk usage with ncurses library." This analysed my /home/$USER directory and didn't show anything near the missing space. On to Plan B!
I then started 'baobab' and running it as 'sudo baobab' and it is "GNOME disk usage analyzer - Disk Usage Analyzer is a graphical, menu-driven application to analyse disk usage in a GNOME environment. It can easily scan either the whole filesystem tree, or a specific user-requested directory branch (local or remote). It also auto-detects in real-time any changes made to your home directory as far as any mounted/unmounted device. Disk Usage Analyzer also provides a full graphical treemap window for each selected folder." This scanned the whole of / and didn't show any sign of the missing 140gigs either.
So where was that 140gigs? It wasn't in /home/$USER and it wasn't in / either! I don't know, I couldn't find it! I'd already deleted 750mbs of cruft which wasn't needed so was safe to delete. My only other option now was to reboot, and pray! Which is what I did!
And when it came back up it showed 140gigs of free space! It was obviously too much to remain in memory so it had been dumped as temporary files onto the hard drive, which rebooting deleted them.
Normal service had resumed!