We don't know anything about your installation... WD version, or Mac Os system, so anything is a guess. Please update your profile.
However, assuming you are Leopard, it seems you have a runaway syslog (system log) There are a number of ways to deal with this, I will give you the most probable fix first

1 Open Terminal in the Utilities folder
2. Type in or copy and paste the following:
sudo launchctl stop com.apple.syslogd
You may be asked for a password, enter it and press return
3. Type in or copy and paste the following:
sudo rm /var/log/asl.db
4. quit terminal, restart your Mac
5. after the restart go to Utilities>Disk Utility and repair the permissions on your Hard Disk
Should be back in business, if not let us know and we can try plan B (There are multiple causes for this) but this is the most widely applicable fix.