Help! screwed-up root partition

I've screwed up in trying to install and use a 1.5TB external HD on my
2008.1 MDV.

Thinking I was writing to the new HD, I wrote some stuff which evidently
went into the / partition (only 3.5G) instead - how I was able to do
this as user I don't know - and filled it up. I moved this out (again as
user and not realising it was in /).

Now 'df' shows the / partition as 100% full and I get warning messages
on boot about inadequate space in the partition.
However, the directories in the / partition (/bin, /dev, /etc, /initrd,
/.kde, /lib, /media, /mnt, /opt, /proc, /root, /sbin, /sys - the rest
have their own partitions) only account for about 10% of the partition
space.

How do I find out where the other 90%'s gone? How do I free it up again?


Doug