The hard disk on my G4 iMac 10.2.8 was getting full, after over keen use of a new camera.
So I got a new La Cie 1TB external drive to replace the old 250 GB drive, thinking I could unload files and make more space.
In the process I saw some warning messages that bad things would happen if I carried on using computer with so little spare space [ a few hundred MB].
There did not seem to be much option so some folders were dragged to TRASH, but now TRASH will not empty.
Worse, Mail will not open as it states there are no accounts.
And trying to do some move/delete activities gets stopped by "insufficient space" error messages.
I don't know if there are any articles in Knowledge Base or elsewhere on how to get out of this mess, but in my opinion, my advice is to never get in the mess in the first place, and make sure you always have at least 1 GB available space on the hard disk.