I found out something annoying about the hard drive icon on the desktop: if you have the Show Item Info option enabled (in View -> Show View Options), then the amount of free space in the info line never gets updated by itself. You either have to quit the Finder, reboot, or log off and on to make it refresh).
I read in some internet forum that copying any file to the top level of the drive (/) would make the available space figure refresh. So what I did was create an empty file in my Documents folder (touch space.txt in the Terminal), and then using crontab and a small script, copy this file every five minutes to /.
Here is the script, which I named freespace_updater.sh:
#!/bin/sh
rm /space.txt
cp /Users/martin/Documents/space.txt /
And the associated crontab entry:
martin$ crontab -l
0,5,10,15,20,25,30,35,40,45,50,55 * * * * /usr/bin/freespace_updater.sh
I know this is very dirty, but at least it refreshes my free space info every five minutes.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050827183424362