I think where he was referring to the root hard drive he in fact meant the boot hard drive not the folder, if you didn't put the "stuff" there then its most likely logs.
If I were you I would be looking at the config file for the main services you run, web servers, mail servers, ftp server etc to find out where they store their log files and purge them (check the correct procedure to do this before you rm -rf them else you may end up disabling the service)
If your running CentOS then I'd run 'yum clean all' as well, this clears yum's cached packages and headers (I found mine to be taking 800Mb today).
To find the largest directory's try this command, it can be invaluable at finding disk space hogging folders:
Code:
du -a / | sort -n -r | head -n 10
For more information on this command goto:
http://www.cyberciti.biz/faq/how-do-...sd-filesystem/