Just as a test we recently backed up my son's home directory from his
macbook to venti.

Some random tool had dropped a symlink somewhere down in his directory
that pointed to /. Result was that all 60G went to venti.

Since that time, the machine has not been happy. Looking at it, the
hard disk is constantly pegged. I have no idea if the two events are
related, but this is a machine
that had been up for 4 months; I had to reboot it just 2 days ago as
it froze on me.

What's a reasonable way to find out what activity is eating the drive?
I have to admit I'm not sure where to go with this.